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Glossary of Methodological Terms

Methodological Terms	Description
Absolute risk	Same as risk. Contrast it with relative risk.
Absolute risk reduction	Risk difference between the control group and the intervention group.
Accuracy	Truthfulness or correctness of a particular value of a measurement to the reality. Age recorded as 7 years 4 months and 14 days is more accurate than recorded only as 7 years, although this additional accuracy may be redundant. Also BMI is more accurate measurement of overall obesity than ecto-, meso-, and endomorph categories.
Adaptive design	A research design that builds flexibility into itself at the time of designing. This flexibility could be adding or deleting a group, changing the outcome of interest, but most often, changing the sample size in each group when interim results indicate the need for doing so.
Addition rule (of probability)	The probability of occurrence of either of two or more mutually exclusive events is the sum of the probabilities of their individual occurrence.
Additive model	A model stating that combined effect of two or more factors is the sum total of their individual effects â€“ valid when there is no interaction.
Adjusted correlation	Same as partial correlation.
Adjusted odds ratio	The odds ratio obtained after eliminating the effect of other concomitant variables that might be affecting the OR. This is generally obtained by including these concomitants in the logistic regression.
Adjusted rate	The net rate obtained after eliminating the effect of other concomitants that might be affecting the rate. This adjustment increases the comparability between rates in different segments of population.
Adjustment	A procedure by which the effect of structural differences in the two or more groups is minimised thus improving the comparability. Common methods of adjustment are regression and standardisation.
Aetiology diagram	A diagram that depicts the inter-relationship of various aetiological factors leading to the disease.
Aetiological factor	The characteristic that contributes to the occurrence of disease or a health condition. It may or may not be a causal factor.
Age-adjusted death rate	See age standardisation.
Age standardisation	A procedure of adjustment to remove the effect of differences in age composition of the groups to increase comparability. This adjustment is required when, for example, one group has older subjects than in the other and the outcome of interest is death. For deaths, this is called age-adjusted death rate. This could be done by direct standardisation or indirect standardisation.
Age specific death rate	Number of deaths of specific age-group in one year per 1000 population of that age-group.
Agreement	When two procedures, two observers, or two sites, etc., tend to give same result in each subject, they are said to be in agreement with one another. For qualitative data the extent of agreement is statistically measured by Cohen's kappa and for qualitative data by limits of disagreement.
Aleatory uncertainties	Uncertainties arising from variation in the factors internal to the system such as biological, psychological, and environmental. Contrast it with epistemic uncertainties.
Allocation bias	The systematic error in results arising from specific allocation of subjects to the test and control group, such as due to nonrandom allocation.
Alpha error	Same as Type I error.
Alpha error rate	The probability of Type I error.
Alpha level	Same as significance level.
Alternative hypothesis	A plausible hypothesis that is accepted when the null hypothesis is rejected.
Analysis	The process of going into the deep of a phenomenon, data-set, thought, etc., and looking at its various components.
Analysis bias (for data)	Gearing data analysis to support a particular hypothesis and ignoring aspects that contradict the hypothesis, e.g., using an inflated P-value for some statistical tests, using proportions when mean is appropriate, etc.
Analysis of variance	Breaking variance into its components such as within groups and between groups. This method is used in regression analysis and in various other situations but more commonly for comparing three or more means.
Analytical study	A study with the objective to identify the determinants or correlates of an outcome, such as aetiological factors, or to delineate their specific contribution to the outcome. See observational study, experiment.
ANCOVA	Acronym for analysis Of covariance: a statistical procedure used when the dependent (or outcome) variable is quantitative, and the independent (or antecedents) variables are a mixture of quantitative and qualitative variables.
Anecdotal evidence	Incidental observation that lacks scientific scrutiny.
ANOVA	Acronym for analysis Of variance: A method for analysis of quantitative out-come when it is dependent on qualitative characteristics, particularly the groups of subjects.
Antagonism	The situation where combination of two or more factors depresses the intensity of outcome compared to the sum of their individual effects: a negative interaction effect.
Antecedent	A characteristic that precedes the outcome. It may or may not be (fully or partially) responsible for the outcome.
APACHE score	Acronym for Acute Physiology And Chronic Health Evaluation: A scoring system that helps to assess the prognosis in case of critical condition of a patient. This is based on state of consciousness, reflexes, eye movements, blood pressure, etc.
Apgar score	Sum total of (0 to 2) scores assigned to each of heart rate, respiration, muscle tone, skin colour, and response to stimulation in a newborn. A low score such as less than 4 out of possible 10 is an indication of poor prognosis.
Area sampling	The method of sampling that uses geographical area as the unit. When the sampling frame of the study units is not available, area sampling can be used in a community-based study to select some areas by using a map, and then all the subjects in those selected areas can be included.
Area under the curve	The area from the ROC curve to the base: used as an indicator of the efficacy of a test in terms of sensitivity and specificity â€“ can be used to compare performance of various medical tests. The area under the concentration curve is used to assess the efficacy of a regimen and to compare performance of two or more regimens.
Arithmetic mean	Same as mean.
Arm of a trial	The case (or test or intervention) group is one arm of a trial and the control group is the other arm. The test arm may have more than one regimen.
Ascertainment (or assessment) bias	Bias due to paying more attention to cases than controls in assessment, or giving more attention to specific outcomes of interest. Also when more accurate history is given by diseased then nondiseased, or when they are more cooperative than the others. Also when outcome of interest is morbidity but there are early deaths that no longer can contribute to morbidity. Assessment bias also occurs when the observer is able to establish more rapport with some respondents than others, e.g., due to cultural similarities. Diabetes and gallstone may appear to be associated because diabetics are regularly checked for gall stone and nondiabetics are not checked.
Association	The property of change in one qualitative factor being accompanied by the change in the other. This change can be causal, incidental, or due to a third intervening factor. The association can be full or partial of various degrees, and can be negative or positive.
Attack rate	New spells during a specified time interval (such as the period of an epidemic) as percentage of the total population at risk in the same interval. See secondary attack rate.
Attributable fraction (of risk)	The proportion of risk that can be validly assigned to a particular exposure.
Attributable risk	The additional risk that can be attributed to the presence of a risk factor. If risk of diabetes in those with both parents diabetics is 0.30 and in those with one parent diabetic is 0.25 (no other difference) then risk attributable to the diabetes in the second parent is 0.30 â€“ 0.25 = 0.05.
Attribute	A qualitative characteristic.
Attrition	Loss of subjects during the course of the study. This can happen due to temporary unavailability, migration, refusal to participate, severe injury, or unrelated death.
Attrition bias	Systematic difference between the groups in the pattern of attrition of the subjects.
Auto-correlation	In a time series, the correlation of values with their preceding value. Auto-correlation of lag 2 is the correlation of values at time t with the values at time tâ€“2. Correlation of monthly values such as of dengue cases with the cases 12 months ago will be auto-correlation of lag 12.
Balanced design	A design in which all groups have same number of subjects (equal n).
Bar diagram	A diagram appropriate for disjoint categories to show the number of subjects or mean or rates by bars of corresponding height.
Baseline data	The data that show the status of the subjects at the initial stage, generally at the time of beginning of the study. The baseline is used for evaluating whether any subsequent change is either due to intervention or some other factor.
Bayes'rule	The rule that converts probability of A given B to the inverse probability of B given A. Most common use of this in medicine is in obtaining the probability of disease given signs/symptoms by first obtaining the probability of signs/symptoms given disease, and in obtaining predictivities on the basis of sensitivity and specificity. Both require some additional information.
Bed population ratio	Number of hospital beds per 1000 population or the number of people per bed in an area.
Before-after study	Assessing subjects before an intervention, and after the intervention to find changes thus assessing the utility of the intervention without using a control group; also called a non-controlled trial. Part of the effect found in this study could be due to psychological factors.
Bell-shaped distribution	Same as Gaussian distribution.
Berkson's bias	Occurs when the study is based on hospital cases but the exposure is such that increases the chance of admission. Thus hospital cases will have more exposure than hospital controls such as fracture in motor vehicle injury cases.
Beta error	Same as Type II error.
Beta error rate	The probability of Type II error.
Bias	A systematic error that can falsify or distort the results of a study. Contrast it with random error. Bias can arise due to a variety of sources: (i) errors from faulty logic or incorrect premises; (ii) nonrandom sample such as volunteers; (iii) small sample that fails to represent the entire spectrum of subjects; (iv) concomitant medication or concurrent disease that might seem unrelated; (v) lack of matching in case and control groups; (vi) unaccounted confounders; (vii) wrong or blurred definitions that give room to assessor to use subjective interpretations; (viii) errors in diagnostic or screening criteria; (ix) insufficient instructions regarding what to do in unusual or unforeseen situations so that observers use their own discretion that can vary from observer to observer; (x) more care in assessment of cases than controls; (xi) differential recall of past events by cases and controls; (xii) not being able to recall events of the past, or selective recall of serious events and not of mild events; (xiii) differential compliance of regimen and instructions by cases and controls; (xiv) suppression of information by patients because of stigma or otherwise; (xv) premeditated response by a group after consulting each other; (xvi) aberration in response due to unsuspecting event such as sudden illness or death in the family; (xviii)some control subjects or some subjects in treatment group getting unaccounted therapy outside the study; (xix)detection of some cases in early phase of the disease and some in the late phase; (xx)prevalent cases could be more of those who survive for longer duration, and thus may be of better health; (xxi)the study is based on hospital cases but the exposure is such that increases the chance of admission; (xxii)observer able to establish rapport with some and not with other subjects due to cultural or other differences; (xxiii)observer being more careful or attentive for specific type of subjects or special responses; (xxiv)faulty instrument that gives unpredictable readings; (xxv)improper use of instrument; (xxvi)systematic error in measurement, which could be either due to the instrument, or due to the observer; (xxvii)subjects changing response just because they are being observed; (xxviii)more accurate history by diseased than nondiseased or better cooperation by one group than the other; (xxix)selective nonresponse; (xxxi)third party or natural introduction of unsuspecting new intervention among the respondents or their habitat that could alter the outcome; (xxxii)contamination of controls by spill-over effect of cases when the two groups are not isolated; (xxxiii)selective handling of outliers; (xxxiv)digit preference; (xxxv)gearing analysis to support a particular hypothesis; (xxxvi)using differential P-values to support a particular view; (xxxvii)lack of statistical power to detect medically important difference; (xxxviii)incorrect interpretation of results; (xxxix)preparing a report to support a particular view by suppressing some facts; (xl)selective publication by journals; and (xli)bias in presentation of results emphasizing one particular aspect and de-emphasizing the other. The list is still not exhaustive. Some of these are explained in more detail under the term for respective bias.
Bibliography	A list of citations of the related literature. This is different from the list of references because references are restricted to the literature actually cited in the text. Bibliography includes references to the other literature as well that are not cited but are related.
Bimodal distribution	A frequency distribution of a variable that has two modes; there will be two distinct peaks although one peak can be very small relative to the other. Age distribution of Hodgkin's disease has a smaller peak around 25 years and bigger peak at 55-60 years.
Binary variable	A characteristics that is assessed only in two categories such as ascites present or absent (or yes/no), or gender as male or female. A qualitative variable is mostly binary (in some cases could be polytomous and/or ordinal) but a quantitative variable can also be made binary by dividing into two categories such systolic blood pressure <140 and Â³140 mmHg.
Binomial distribution	The distribution that gives the probability of occurrence of x successes out of n where the probability for each is the same. If the chance of hypertension is the same 20% in males of age 40-49 years, the probability that exactly 6 out of 10 coming to a clinic have hypertension is (10C6) (0.20)6 (1-0.20)10-6, by binomial distribution.
Bioequivalence	Similar course of the disease process in the two regimens under comparison: also evaluated in terms of comparable bioavailability of drug products, say, within 80% to 125% with respect to area under the curve and Cmax. Also see equivalence and therapeutic equivalence.
Biological plausibility	Consistency with the present biological knowledge, which can be explained.
BioSIS	The UK-based citation service that processes articles from a large number of journals, books, monographs, conference proceedings, etc., on all topics of biological sciences.
Biostatistics	The science dealing with medical uncertainties in a group of subjects
Birth cohort	A cohort of people which is being followed since births â€“ all births taking place at a specified time period.
Birth rate	Number of births in one year per 1000 population.
Biserial correlation	The correlation between a quantitative variable and a dichotomous variable such as between sex and creatinine level.
Bivariate analysis	A statistical analysis of data by considering two variables together, such as maternal haemoglobin level and birth-weight category, or alcohol intake and occurrence of liver cirrhosis. One or both variables can be either quantitative or qualitative.
Black box approach	Using computer to solve problems without understanding the implications of the underlying procedure.
Blinding	Keeping the experimental subject or the observer or both ignorant about which subject is in the case group and which in the control group in a trial.
Body mass index (BMI)	Weight in kg divided by square of height in meters: a measure of obesity in adults. A BMI <20.0 kg/m2 is considered lean, 20.0-24.9 is considered â€˜normal', 25.0-29.9 overweight, and Â³30.0 obese.
Bonferroni procedure	When two or more comparisons or other statistical tests of hypothesis are done on the same set of data, the total probability of alpha error can increase much beyond the prefixed level such as 5%. In order to keep the error probability within the specified level a, the Bonferroni procedure is to do individual comparisons at a/k level of significance where k is the total number of comparisons. If k = 4 and a = 0.05, each comparison is done at 0.05/4 = 0.0125 level. This is a conservative procedure in the sense that the total level of significance is actually less than a.
Bootstrapping	A computer based data resampling method that helps in estimating the sampling variance and other features
Box-and-whiskers plot	Same as box plot.
Box plot	A diagram that shows the median, the first and the third quartile (the difference between them giving an idea of dispersion, and the distance from median on either side, of skewness), and the lowest and highest value after excluding outliers: a very effective method to present so many features of data in one diagram.
Bubble chart	A chart that depicts the varying quantity of a third variable with different values of two quantitative variable. How infant mortality (MR) changes as the time advances and income changes can be depicted by bubble chart where year can be on x-axis, income on y-axis and bubbles various sizes plotted as scatter: their size in proportion of the IMR.
Burden of disease	A composite measure of premature mortality from a disease and morbidity equated to mortality through a weighting system based on age, discounted duration and severity of disease. Premature mortality is assessed in comparison to the mortality in the population with highest life expectancy.
Butterfly effect	An effect that blows out of proportion in course of time but starts from a minor effect. For example, a minor difference between two cancer patients at the time of detection may end up in quick proliferation in one and small changes in the other.
Capture-recapture method	A method to estimate the size of the population by the extent of overlap in two independent sources from the population. Under this method, one sample is taken and those with a particular characteristic are marked. The entire sample is replaced back and allowed to randomly mix with the population. Now another sample is taken, and the number found with the mark are used to estimate the size of the population. This method is commonly used to estimate, say, tigers is the wild but is used in medicine to estimate the number of patients in a population.
Case	A person or unit of interest possessing a specified characteristic, such as a person with the disease or a family living in the conditions of interest.
Case-control study	Investigation of the antecedents in a group of cases and equivalent controls without introducing any intervention. The groups are defined on the basis of presence or absence of disease or any other outcome of interest. The logic of the design leads from effect to the cause. All case-control studies are inherently retrospective.
Case-fatality rate	The number of cases who die out of those who are suffering from a particular disease. This measures the virulence of the disease. Case-fatality in typhoid is low and high in tetanus. For chronic diseases such as cancers, the case-fatality may be 100% but measures such as 5-year death rate may be better indicators of the â€˜virulence' of disease.
Case group	The group of subjects that already has the disease or the condition under study.
Case-referent study	Same as case-control study but comparison is with some other disease and not placebo.
Case series	A specific group of patients with the disease of interest; generally consecutive cases reporting in a clinic, or observed in a community. There is no control group in this setup. Case series is one method of a descriptive study.
Case study	Study of one individual, particularly with regard to the antecedent and outcome factors as observed in that person.
Categorical data	All qualitative data are categorical. In addition, quantitative data are also summarized into categories. For example, for smoking index of 300 individuals, the table may have categories such as 0, 0.1-4.9, 5.0-9.9 and 10.0+. Sometimes such categories are used for analysis and inference purposes also.
Cause-effect relationship	Statistically significant dependence of an outcome on an antecedent so that any change in antecedent makes corresponding changes in the outcome when all confounders are absent. The relationship should also meet other criteria such as temporality, consistency and biological plausibility. See necessary cause and sufficient cause.
Cause-specific rate	The rate obtained when numerator is restricted to a particular cause (e.g., of morbidity or of mortality). Cause-specific death rate is the number of deaths due to a cause per thousand population. Sum of cause-specific death rates for all causes in the same as crude death rate.
Census	Survey of the entire population.
Centiles	Same as percentiles. See also deciles, quartiles, tertiles.
Central limit theorem	A statistical result that says that sum of a large number of values from any distribution tends to have a Gaussian distribution.
Central tendency	Among variations, there is still a tendency for a set of values to gather around a central value. Mean, median and mode are measures of central tendency.
Chance	1. Colloquial name for factors that are unknown or too complex to comprehend such as a collision occurring by chance. 2. Colloquial term for probability.
Chi-square test	A versatile statistical procedure that is used to test different types of hypothesis on proportions, such as equality, trend and relationship.
Circular sampling	A systematic method of sampling that starts from any number and not necessarily from the beginning. Under this sampling, the (N + 1)th unit for selection is the first unit. This method removes some demerits of systematic sampling.
Citation	Identification data of a document containing the authors'name, title, publication name, volume, publication date, page numbers, etc.
Classification	Placing a unit to one of the two or more known classes or categories. Units within each category share some similarity among them, and units in different classes are dissimilar.
Clinical agreement	See agreement.
Clinical epidemiology	Application of principles of epidemiology to individual subjects, particularly to the patients.
Clinical equipoise	Genuine uncertainty among the experts about the relative merits of the regimens under trial: thus no research group is particularly disadvantaged.
Clinical significance	A situation where a result is capable of modifying the management of a patient.
Clinical thresholds (of normal range)	A range of values of a quantitative medical measurement in healthy subjects that has least overlap with the values found in diseased subjects so that the chance of misclassification is minimum. But the chance of error remains. For example, for blood pressure this is 140/90 mmHg.
Clinical trial	A medical experiment on human subjects, particularly in a clinic setup, such as to find efficacy and safety of a new therapeutic or diagnostic regimen.
Clinimetrics	Assigning scores to clinical entities for diagnostic or rating purposes
Close-ended question	A question for which list of possible answers is already provided.
Cluster	A group of subjects with some commonality
Cluster random sampling	Dividing the target population into clusters of specified size and selecting a few clusters by random method.
Cochrane Collaboration	An international organisation of producers and consumers of medical research that helps to clarify the research achievements, particularly health care interventions such as drugs, diet alteration and behaviour change. The focus is mostly on systematic reviews or correct meta-analysis of the relevant studies.
Cochrane Review	A review of trials, mostly based on meta-analysis, following specific guidelines of Cochrane Collaboration.
Coding	Assigning a numeric to a qualitative characteristic, such as code 1 for hypertension, code 2 for diabetes, code 3 for cancer, etc. Codes are not quantities, and care should be exercised that they are not used as quantities at the time of analysis. These are used only for convenience in data entry.
Coefficient of determination	The percentage of total variation in a variable explained by one or more of the others. In a simple linear regression setup, this is square of the correlation coefficient. In a multiple linear regression setup, this is square of the multiple correlation coefficient.
Coefficient of variation	Standard deviation divided by mean. This unit-free measure is used to compare dispersion of one variable with the other, such as dispersion of cholesterol level with dispersion of body temperature in a group of cases.
Cohen's kappa	For qualitative data, a measure of agreement in excess of chance between two or more observers, methods, sites, etc.
Cohort	A clearly defined person or a group of persons with some common feature, who are followed for an outcome in a specific period beginning from a defined common baseline: not necessarily beginning at the same time. A cohort of women taking contraceptive pill may start from the day of first intake but it may include women starting in any chronological month of the year. See also inception cohort.
Cohort study	A prospective study of a cohort for a specified period, generally to observe the occurrence of an outcome of interest, and thereby determine the incidence and risk.
Collation of data	The process of rearranging the data into intelligible form so that either the conclusions can be drawn, or analysis can be done.
Community trial	Same as field trial.
Completely randomized design	A design under which the subjects are allocated completely at random to different groups with no consideration of any factor such as age and sex.
Compliance bias	Either higher noncompliance by treatment group relative to the control because of discomfort or poor intake of the drug, or better compliance by them because they are improving.
Concealment of allocation	The process of allocation of subjects to the groups that is impervious to any influence of the person making the allocation. Among methods of such concealment are centralised randomisation without participation of the observer, coded and identically looking packing of the placebo and drug, and sequentially numbered opaque envelopes.
Concentration curve	Plot of quantitative response versus time such as of concentration of drug in body at different points of time after intake.
Conceptual bias	Errors arising from faulty logic or incorrect premises.
Concomitant variable	Same as confounder.
Concordance	Agreement or similarity between two individuals in a paired setup.
Concurrent validity	Consistency of response to two or more questions in the sense that they reflect the same pattern. For example, high calorie intake and low exercise together should correspond to greater obesity. Response to these three items should be consistent with one another.
Conditional probability	The probability of occurrence of an event such as disease when some a-priori information such as sign-symptoms are known: denoted by P(A/B) where after slash (/) sign is what is known a-priori.
Confidence bound	A one-sided confidence interval.
Confidence interval	The interval within which results of other similar studies is expected to lie with a certain confidence level.
Confidence level	The degree of assurance that other studies of similar type will have the same results as obtained by the participants in the current study.
Conflict of interest	Personal, financial, or other interest of any investigator that could influence the finding or the interpretation.
Confidence limits	The upper and lower boundaries of a confidence interval.
Confounder	An extraneous factor that could be a full or partial explanation of the outcome of interest in addition to the factor under study so that its effect can not be differentiated from the other: such as dietary factors when examining relationship between smoking and cervical cancer. Presence of unaccounted confounders decreases the validity of a study.
Confounder bias	Bias due to presence of one or more unaccounted confounders.
Consecutive sample	The sample of subjects enrolled on the basis of the sequence of their arrival, and none is excluded if eligible till the desired number is reached.
Consistency	Same as reliability.
Consort	Acronym for CONsolidated Standards Of Reporting of Trials: This provides guidelines on how a trial results should be reported.
Construct validity	The ability of a set of items or questions to assess a given theoretical concept. Suppose positive health is defined as the ability to withstand physical stress, and the intention is to measure it by excess in haemoglobin level, forced vital capacity, and pain bearing capacity. The ability to withstand stress is also separately measured by items such as lack of restriction in daily activity by injury or fever. The correspondence between these two assessments will indicate construct validity of the measurements. It is the agreement between the theoretical concept and the specific device used to measure that concept.
Content validity	Sufficiency or adequacy of the items or questions to measure a phenomenon. For example, spinal palpatory test may not have good content validity to identify spinal neuromusculoskeletal dysfunction. Content validity is judged by a panel of experts.
Contingency table	A table containing the number of subjects with different characteristics, which should be mutually exclusive and exhaustive, such as number of subjects with and without disease, and each with a positive or negative test. A contingency table is used to test if one characteristic is associated with the other.
Continuous variable	A variable that can theoretically have infinite number of possible values within a short range. Age is continuous since within 8 and 12, it can be 8.17, 10.874, 9.756 years, etc. Age can be measured in terms of days, hours and minutes, although practically there is no need to do this. Blood pressure is a continuous variable but measured in integers for convenience. Parity is not a continuous variable because there is no possibility of it being 2.75 or 1.6.
Control (group)	Used in two senses: 1. The group of subjects that do not receive the test regimen. They may receive placebo, or the existing standard regimen, or any other regimen that is appropriate for comparison. 2. The group without disease or without any other outcome of interest.
Control (programme)	A defined series of steps to reduce or eliminate a disease.
Control (programme)	A defined series of steps to reduce or eliminate a disease.
Control (statistical)	The statistical process of adjusting for any extraneous influence on the results.
Control (subject)	A person or unit of interest but not possessing the specified characteristic, such as a person not having the disease of interest, or a person being treated by regimen other than under test.
Controlled trial	A trial that compares intervention group to a control group: when not further qualified this generally indicates a trial with nonrandom or quasi-random allocation of subjects to the test and the control group.
Convenience sample	The group of subjects that are selected primarily because they were available at a convenient time or place. This is one of the several ways that a purposive sample can be drawn.
Correlates	Factors that are related in some way to an outcome of interest. They may or may not be contributors to the outcome.
Correlation	The degree or strength of relationship between two quantitative variables. Loosely used for qualitative variables also. For linear relationships, it is measured by Pearsonian correlation coefficient that ranges from â€“1 to +1. A negative correlation means that increase in the value of one variable is accompanied by linear decrease in the other, and vice-versa. A positive correlation means that the two move together in the same direction. A correlation close to zero means that increase or decrease in one does not linearly affect the other. Correlation coefficient can be close to zero when a strong relationship is present but is nonlinear.
Correlation coefficient	See Pearsonian correlation coefficient. The other types of correlation are Spearman's, multiple, partial, etc.
Cost-benefit analysis	The assessment of benefit per unit of cost when both are measured in monetary units.
Cost-effectiveness analysis	The assessment of effectiveness (life saved, disability restricted, year of life gained, etc.) per unit of cost.
Covariance	A measure of how the product of two quantitative variables behave
Covariate	Same as confounder: generally used in a restrictive sense for quantitative variables only.
Cox regression	A type of regression that models logarithm of hazard ratio on the covariates that affect this ratio. Also see proportional hazards model.
Criterion standard	Same as gold standard, but a preferred term.
Criterion validity	Ability of a device to provide a measure that correlates or agrees with the criterion known to correctly measure the characteristic of interest.
Critical value (in the hypothesis testing)	The threshold value of the statistical test criterion such as c2, t and F, beyond which it is considered statistically significant.
Cross-over design	A design that stipulates that same subjects will get the test and the control regimen after a washout period, but the sequence is randomised. Half the subjects get regimen A followed by B, and the other half B followed by A. Using same subjects for both the regimens reduces variability and thus also the level of uncertainty in the results.
Cross-product ratio	Same as odds ratio.
Cross-sectional study	An analytical study with a format that elicits information on the antecedents and the outcomes at the same time. Such a format is poor to investigate cause-effect type of relationship but is good to generate hypothesis.
Cross-tabulation	The process of obtaining the number of subjects of various types when divided by two characteristics simultaneously such as distribution of myocardial infarction cases by their lipoprotein(a) and homocysteine levels. In most cross-tabulations, these levels would be in categories such as 10.0-14.9, 15.0-19.9, etc.
c-statistic	A statistic that measures the area under an ROC curve.
Crude death rate	Total deaths in one year in a population divided by mid-year population.
Current Contents	The electronic database of table of contents and bibliographic citations from current issues of more than 7500 research journals in sciences, social sciences, arts, and humanities.
Curve	Opposed to straight line, the curve depicts relationship that varies at different values of two variables. For example, one variable may increase for lower values of the other and then decrease for the higher values of the other. Another example is the relationship between glomerular filtration rate (GFR) in chronic renal failure cases and plasma creatinine level. As creatinine level rises, GFR declines steeply in the beginning and then declines slowly.
Curvilinear relationship	A relationship that is depicted by a curve opposed to a line but can be converted to a line through some mathematical transformation.
Data	A set of observations, generally in numerical format but can be in text format also (plural of datum).
Database	A collection of items of data arranged in a predefined format, usually held on a computer.
Data cleaning	The process of correcting and deleting the incomplete or apparently wrong observations from the data set. Sometimes this may suggest a relook at the original source for finding the correct value.
Data dredging	Initially used for excessive analysis of data in search of new hypotheses, but now used for re-analysing data after deleting some inconvenient values so as to fit them into supporting a particular hypothesis
Data editing	Same as Data cleaning.
Data Safety and Monitoring Board (DSMB)	An independent body of reputable persons who agree to monitor a research project regarding the integrity of data and adherence to protocol.
Death spectrum	Variety of causes of death: some people meet death slowly such as by cancer, and some sudden such as by myocardial infarction.
Deciles	Nine cut-points of values of a variable that divide the total number of subjects in ten equal parts: obtained after arranging the values in ascending order.
Decision analysis	The process of reaching to a decision after considering probabilities of various outcomes and value judgments regarding the utility of those outcomes. Decision analysis is considered a very effective method to take a valid decision under conditions of uncertainties.
Deduction method	The method of reaching to a conclusion for a particular individual on the basis of a known generalised result
Degrees of freedom	The number of observations in a dataset that can freely vary once the parameters have been estimated. This concept is used in chi-square, Student's t and other statistical procedures since their distribution depends on degrees of freedom.
Definition bias	The bias due to 1. wrong or substandard definition such as of impotence on the basis of erectile dysfunction alone; or 2. blurred definition that gives room to assessor to use subjective interpretation such as blood pressure Â³140/90 for hypertension without specifying what to do if systolic level is higher and diastolic is lower. Errors in diagnostic or screening criteria also come under this category.
Delphi method	A less scientific but a quick method to arrive at a consensus among experts. In this method, the responses from a panel of experts are iteratively obtained that are progressively refined by reducing the options in successive rounds depending on what options are preferred in the previous rounds.
Demography	The science that studies characteristics such as age-sex distribution, urban-rural distribution, trend over time, etc., of a population.
Dependent variable	A variable that is sought to be explained by one or more of the other variables. The dependent variable is generally the outcome of interest whereas independent variables are the antecedents.
Descriptive analysis	Calculation of simple statistics such as mean, median, SD, number of subjects in different categories, etc.
Descriptive study	A study with the objective to delineate the distribution of disease or a health condition in a defined population, or to describe clinical features of specific kind of subjects. It also includes estimation of parameters such as mean and percentiles. This does not examine causality or aetiology. A descriptive study could be a census, a sample survey or a case series.
Design of experiments	See experimental design.
Design (of research)	The format of collection, compilation, and analysis of observations. See descriptive study, analytical study. Also see sampling design.
Design bias	A design of a study where the selection of subjects is not random, control group is not adequately matched, definitions of subjects of characteristics to be studied are loose, confounders are not properly accounted, etc.
Design effect	The effect on the variance of an estimate, and thus on efficiency of a study, due to the design of the study, such as of cluster random sampling relative to the simple random sampling.
Determinant	A factor that is responsible, fully or partially, for an outcome.
Diagnostic score	A numerical score used as an aid in establishing diagnosis, such as thyroid scoring system to distinguish hypo-, eu- and hyperthyroidism on the basis of clinical assessment.
Diagnostic test	A criterion used for confirming the presence of disease or a condition. This should have high positive predictivity. The criterion could be a laboratory test, a radiological test, or a clinical observation.
Dichotomous variable	Same as binary variable.
Digit preference	Preference for certain digits such as 0 and 5. Then they become predominant in reporting and recording. This can affect the result.
Direct standardisation	A procedure of adjustment to bring the differential structure of two or more groups to a common (standard) base to increase their comparability: most commonly done for differential age structure that can easily affect mortality. In direct standardisation the observed subgroup specific rates are used on the standard group structure.
Disability-adjusted life expectancy	Life expectancy adjusted for equivalent life lost due to disability of varying degrees during the life time.
Disability-adjusted life years (DALYs) lost	The loss of years of life due to premature mortality compared to the most healthy population, plus loss of equivalent years of life due to various disabilities of different severity and duration from morbidities or otherwise. This measures the burden of disease in a population. The disability arises from sickness from time to time.
Discrete variable	A variable that can take only finite, practically small, number of possible values. Parity for a woman is a discrete variable. Deaths in an area with population 200,000 is theoretically discrete but can be considered continuous for statistical purposes because it can take a large number of possible values.
Disease spectrum	Among many types of disease spectra, the one of special epidemiological interest is the proportion of population that is susceptible, proportion that has infection (apparent + inapparent), proportion that has disease, the proportion that has serious form of disease, and the proportion that die. Note that the subsequent proportion is nested in the preceding proportion. Disease spectrum helps to plan the research accordingly, and to take measures to control the disease.
Disease thresholds (of normal range)	The range of values of a quantitative measurement beyond which there is a considerable risk of presence of disease or occurrence of disease. The chance of misclassification remains in this threshold also as in any other such threshold.
Dispersion	The degree of variability or scatteredness of the values of a variable when measured for different subjects or at different times.
Dissertation	A detailed discourse or treatise on a particular topic providing it a new perspective: generally the written work submitted by a candidate for the award of a doctoral (PhD) degree.
Distal measures of health	The background characteristics that indirectly affect the health condition under consideration. Socioeconomic factors such as education, income and occupation make an impact on diet, hygiene and exercise that in turn affect the pathophysiological parameters, on the basis of which a condition is assessed. Thus socioeconomic are distal and pathophysiological parameters are proximal measures.
Distribution (statistical)	The pattern of values when obtained for a large number of subjects. For incubation period of AIDS, the distribution would tell us how many or what percentage of cases have incubation period between 5 and 6 years, what percentage between 6 and 7, 7 and 8, etc. Thus, the distribution tells what has been the dispersion and where has been the concentration of values. See Gaussian distribution, skewed distribution.
Dose-ranging trial	A clinical trial in which two or more doses of an agent are tested against each other to determine their relative efficacy and safety.
Dose-response relationship	Any kind of relationship between quantity of dose and the degree of response but generally indicating that higher the dose, higher the response, such as between smoking and lung cancer.
Double-blind trial	A trial in which neither the experimental subject nor the assessor knows that the subject has received the test regimen or the control regimen. This removes the possible bias in response and ascertainment.
Dropout	A subject who is initially enrolled for a study but whose subsequent measurements as required under the study protocol could not be obtained.
DSMB	Short for Data Safety and Monitoring Board.
Dunnett's test	A statistical test used to compare means in two or more test groups with mean in the control group.
Ecological fallacy	The fallacy that arises from application of group characteristics based result to individuals.
Ecological study	A study in which broad, between-population differences in patterns of exposure to a particular factor are compared with the incidence rates of the disease of interest in those populations.
Effectiveness	The extent to which a regimen is effective in meeting its objectives in actual field conditions or routine circumstances
Effect size	Magnitude of effect of a factor on an outcome. This could be proportion, difference, odds ratio, measure of association, etc. This could also be standardised mean difference between the test and control group obtained as the mean difference divided by its standard error.
Efficacy	The extent to which a regimen is effective in meeting its objectives in ideal conditions
Efficiency	The frequency of desired outcome per unit of resource inputs such as time, money, and manpower. For example, coronary angiography using 4 French catheters may be more efficient procedure than 6 French catheters because of early ambulation without sacrificing the quality of images. Case-control studies are considered more efficient, particularly for rare outcomes, than prospective studies because of lower cost. Campaign against smoking may be more efficient for reducing deaths or for increasing life-years than treatment of lung cancer patients.
Electronic resources	Resources available in electronic format such as on compact disk (CD), diskette, and particularly the world wide web (www). For literature, these include books, journals, reports, other documents, citation databases (e.g., PubMed), etc.
EMBase	Europe-based Excerpta Medica electronic dataset of medical and health research literature covering 5000 journals from more than 100 countries.
Empiricism	The system based on observations and evidence, opposed to theories.
Endogenous factor	Factors within the model under consideration. See also exogenous factor.
Endpoint	The outcome that a study is designed to assess. The examples are recovery, pain relief, duration of hospital stay, side-effects, disease progression, and death.
Epidemiology	The study of factors that affect distribution and determinants of disease or a health condition in a human population.
Epidemiologic consistency	The correspondence among the incidence, prevalence, duration of disease, mortality, etc., so that their underlying relationship is properly reflected.
Epistemic uncertainties	Uncertainties arising from limitation of knowledge and biases. Contrast it with aleatory uncertainties.
Equator network	Acronym for Enhancing, QUality And Transparency Of health Research network that contains details of how various kinds of medical research should be reported.
Equipoise (subject)	Subjects are such that the outcome of test or control regimen is uncertain, i.e., the subjects are not chosen in a manner that they favour one regimen or the other
Equipoise (clinical)	See clinical equipoise.
Equivalence	Difference between two or more groups or regimens by not more than a prespecified clinically irrelevant amount. Two regimens are considered therapeutically equivalent if their efficacy does not differ by more than a prespecified small amount. They are considered bioequivalent if the course of the disease or recovery in the two regimens is nearly the same. Therapeutic equivalence considers only the outcome whereas bioequivalence considers the entire course.
Equivalence trial	A clinical trial with the objective to examine the equivalence of two regimens.
ERMed	Short for Electronic Resources in Medicine: A database of literature (more than 1200 medical journals) being promoted by National Medical Library (India) for medical researchers in the country.
Error	See bias, false positive, false negative, random error, Type I and Type II error.
Error in research (honest, negligent and deliberate)	Honest error is absolutely unintentional that arises from limitation of knowledge, such as not taking care of unforeseen bias. Negligent error is gearing up investigations or findings to support a particular view neglecting the other evidence. Deliberate error is misconduct of plagiarism, reporting inflated sample size, cooking up the results, etc.
Ethics	See medical ethics.
Etiological factor	See aetiological factor.
Etiological diagram	A diagram that shows relationship between the antecedents and outcomes by arrows for direction of the effect. This generally includes the distal and proximal factors as well as confounders.
Evidence-based medicine	Using evidence as available in literature, in records, or in newly generated data, for managing patients after proper accounting of risks and benefits.
Evidence (levels of)	The strength or validity of evidence: a randomized double blind control trial is considered as the level-1 evidence as it is almost foot-proof when the trial is conducted with sufficient care. On the other hand, ecological studies provide evidence of poor qualify that can be easily refuted.
Exclusion criteria	The set of conditions presence of which will exclude an otherwise eligible subject from the study. Generally these conditions are indicative of severe form of disease or complications that render a subject unsuitable for that research. Exclusion criteria are part of the case definition that delineates the target population. The other part is inclusion criteria.
Exogenous factor	Factors that affect the outcome from outside and are not part of the model under consideration. See also endogenous factors.
Expectation of life	Same as life expectancy.
Experiment	A study of effect of intentionally introduced intervention or alteration of a factor, under controlled conditions. In medicine, the term is generally used for laboratory experiments. See trial.
Experimental design	The structure of an experiment in terms of inclusion and exclusion criteria for subjects, method of allocation of subjects to intervention or other groups, and number of subjects in various groups. Sometimes this also includes details of assessments, reliability and validity of tools and of measurements, etc.
Expert system (medical)	An intelligent computer aid to diagnosis, treatment and prognosis. It is capable of storing all the information, and more importantly to selectively retrieve the relevant information as an aid to the physician, and is capable of suggesting a spectrum of diagnosis for the given set of complaints, examination results, laboratory findings, etc., and also suggests a possible line of treatment and the prognostic implications.
Explanatory model	A model that explains how an outcome might have occurred on the basis of other factors. The factors in this model must be biologically relevant. See also predictive models.
Explanatory trial	A trial done under near ideal conditions with practically no deviation. Contrast it with pragmatic trial.
Explanatory variable	Same as independent variable.
Exploratory analysis	A method for examining the patterns and features of a datasetâ€“ generally based on graphical display.
Exploratory study	A small-scale study of relatively short duration, which is carried out to gain baseline knowledge about a problem for which little is known.
Exposure factors	The conditions to which a group is a suspected to have been exposed, which may alter the risk of outcome of interest.
External validity	The generalisability of a result to those subjects that are not included in the study, i.e., the applicability of results to a larger population and not just to the study participants.
Face validity	The property of a measurement or an instrument to apparently look right or reasonable.
Factor	A characteristic that can, or suspected to, cause alteration in an outcome.
Factorial design	A design that includes all possible combinations of the antecedent factors under study. If there are three antecedent factors for visual acuity in adults
Fallacies	See statistical fallacies.
False negative	A person with disease classified as without disease. In place of disease, false negativity can be for any other attribute.
False positive	A person without disease classified as with disease. In place of disease, false positivity can be for any other attribute.
Field trial	An experiment on human subjects in a community, such as vitamin A supplementation to children less than 3 years for examining improvement in their nutrition status.
File-drawer effect	When a draft of a research paper is kept in a drawer for long time, many deficiencies are noted when taken out. These deficiencies occur either because of new learning about the topic or the language, or because of change in attitude towards the problem.
Fisher's exact test	A statistical test used for 2Ã—2 contingency table to find statistically significant difference when the number of subjects is small. When the number is large, this is approximated by chi-square.
Fixed effects model	A statistical model that stipulates that the levels of factors under study are the ones of interest. Contrast it with random effects model.
Focus group discussion	A popular method of qualitative studies in which a cross-section of informed group of people is encouraged to discuss a specific problem. When properly guided and annotated, such a discussion may lead to an effective solution.
Follow-up study	Same as prospective study.
Food-frequency questionnaire	A form which includes a list of 100-150 commonly consumed foods and provides space to fill-in how often each food is eaten. It may also include information on serving-size, variation by season, etc.
Force of mortality	Hazard of death at an instantaneous point of time. In an air accident, the force of mortality is exceedingly high. In home it is very low. The force of mortality in leukaemia is high relative to breast cancer.
Four-fold table	A contingency table with two rows and two columns so that the total number of cells is four (excluding the totals).
Frequency	Used in two senses: 1. Frequency of occurrence per unit of time (per month, per year, etc.). 2. Number of subjects with a particular characteristic or with values in a particular interval, such as how many have homocysteine level between 10 and 14 Âµmol/l.
Frequency curve	A graphical representation of the frequency distribution by a smooth curve.
Frequency distribution	A statistical distribution of subjects that displays the number of subjects with different levels of measurement, e.g., how many have diastolic blood pressure <70 mmHg, how many between 70-74, 75-79, etc.
Frequency matching	Same as group matching.
Frequency polygon	A graphical representation of the frequency distribution by a polygon shape.
Friedman test	A nonparametric test for comparing central tendency in more than two groups in a two-way design.
F-test	A statistical procedure for test of hypothesis on equality of three or more means, and for other setups such as regression.
Gantt chart	A chart that shows time-line of a project: the duration and dates of beginning and ending its various phases.
Garbage-in, garbage-out syndrome	The tendency of getting poor output or poor outcome when the inputs or efforts are poor.
Gaussian distribution	Distribution of values of a quantitative variable such that they are symmetric with respect to a middle value with same mean, median and mode, and the frequencies taper-off rapidly in a particular manner on both sides
General linear model	A model that describes a dependent variable as a linear combination of a set of independent variables with two important features â€“ 1. the variables could be qualitative or quantitative or mixture, and 2. the variables could be square, cube, logarithm, or any such function of the original variable. ANOVA, ANCOVA and regression are special cases of general linear model.
Geometric mean	nth root of multiplication of n positive values.
g-index	A measure of quality of a researcher based on the papers cited by others. A g-index = 15 means that top cited 15 papers of a researcher have been cited at least 152 = 225 times. Also see h-index.
Gold standard	In medical assessment, a method, procedure, or a measurement with almost 100% sensitivity/specificity or 100% predictivity: an extremely difficult proposition to achieve. As a compromise, the best available is considered â€œgoldâ€, against which the newer tests are compared. Thus the better term is criterion standard. This need not be a single or simple procedure but could include follow-up of subjects.
Goodness of fit	How well the actual observations fit into a specified pattern. The goodness of fit is statistically tested mostly by chi-square method.
Group matching	See matching.
Group sequential design	A research design under which pre-decided number of cases are sequentially added in groups when so required to get convincing evidence of efficacy of the regimen or of its futility.
Grouped data	Quantitative values in categories such as age into 0-4, 5-9, 10-14, etc.
Guttman scale	A binary (yes/no) set of questions are on Guttman scale when they follow a hierarchy so that â€œyesâ€ to any item implies that the answer is yes to all the subsequent (or preceding) items. Visual acuity follows Guttman scale since if one can read font size 24 from a distance of 14 inch this implies that she/he can read any bigger font size from the same distance.
Half-life	The duration at which time-dependent outcome or response is 50%. Half life of a drug could be few hours when its availability in the body is one-half of what was ingested.
Haphazard sampling	A mixture of convenience, volunteer, snowball sampling, etc., that does not follow any specific procedure.
Harmonic mean	Reciprocal of the mean of reciprocals.
Hawthorne effect	The tendency of subjects changing their response when they know that they are being observed. This can happen with the test group as well as the control group.
Hazard rate	The force of occurrence of events at a (instantaneous) point of time, such as force of mortality or of morbidity when measured per unit of time. This could exceed one.
Health situation analysis	An exercise to assess the health condition of the people in an area, availability and utilization of health infrastructure, expenditure on health, etc.
Healthy life expectancy	The remaining portion of life at any age that would be spent without any morbidity. Life expectancy at age 40 may be 36 years but healthy life expectancy would be only 30 years if 6 years on average are spent in nonhealthy states as per the current pattern of morbidities.
Helsinki Declaration	A set of ethical rules to plan, conduct and report a study on human subjects.
Herd immunity	When a large percentage of susceptibles such as more than 90% is immunized, the infection feels strangulated, as it is not able to find susceptibles in its proximity. Thus the infection fails to spread.
Heterogeneity	Variability or differentials among measurements, subjects, specimen, results, estimates, etc. Contrast it with homogeneity.
h-index	A measure of quality of research of a person based on the citations of his work. An h-index = 12 means that his 12 papers have received at least 12 citations each. See also, g-index.
Histogram	A diagram showing the number or percentage of subjects with values in different intervals by means of contiguous bars: used only for quantitative variables.
Historical cohort	A cohort whose common baseline is in the past, and mostly outcomes too have already occurred. But the format of investigation still is from antecedent to outcome. The past is reconstructed mostly on the basis of records or recall.
Historical control	A control group or a subject for which information was collected earlier than the group being currently studied. This information can be biased because of changes over time in risk pattern, techniques, concepts, etc.
Homogeneity	Similarity among measurements, subjects, specimen, results, estimates, etc. Contrast it with heterogeneity.
Homoscedasticity	Equality of variance of values in different groups.
Hypothesis	A statement of belief that is made before the investigation regarding the status of parameters under study, including those that measure relationship. See null hypothesis, alternative hypothesis.
ICD	Short for International Classification of Diseases.
Iceberg phenomenon	When there are a large number of undetected cases for each detected case, such as in the case of HIV infection. Also when one clinical case means many infected but inapparent cases such as for AIDS.
Impact factor (of a journal)	The average number of times articles from a journal are cited by others in preceding two years
Imputation	The process of filling in of plausible values for missing data.
IMRaD format	A format for writing research articles
Inception cohort	A cohort assembled at the beginning phase of disease or a condition that is followed up to examine its course.
Incidence	The number of cases newly occurring or arising over a defined period of time.
Incidence density	Same as incidence but now number of new spells are counted instead of persons newly affected. One person can have more than one spell of diseases such as diarrhoea and angina in a defined period.
Incidence rate	Incidence per unit of time and per unit of population. If incidence of benign prostatic hyperplasia in a population of 1,00,000 adults in six months is 16 cases, the incidence rate is 0.32 per 1000 adults per year. This can also be calculated per person-year or per 100 person-years, etc.
Incident cases	Same as incidence.
Inclusion criteria	The set of characteristics such as age, disease and severity, which are necessary in a subject to be considered eligible for inclusion in the study. Some of these subjects may become ineligible when exclusion criteria are imposed. The inclusion and exclusion criteria together define the study subject and the target population.
Independence	If occurrence of one event does not affect the occurrence of the other, they are called independent. Body temperature is generally independent of blood pressure levels but not of heart rate. Occurrence of typhoid is independent of colour blindness
Independent variable	A variable that is used as an explanation of another variable. Independent variables are mostly the antecedent factors that affect or suspected to affect the (dependent) outcome. Some of these variables can be manipulated by the researcher to alter the outcome.
Index	A composite of two or more indicators, such as body mass index (BMI) for obesity that combines height and weight, and Indrayan's smoking index for a combination of age at initiation, duration, quantity and type of smoking, and the time elapsed since quitting by exsmokers.
Index case	An affected person who might affect others also. In the case of infection, the infected person is an index case who can spread the disease. When a new person is infected, he can be an index case for other susceptibles.
Indicator	A single measurement that indicates the existence or magnitude of a condition. Signs-symptoms are indicators of a disease, smoking an indicator of lung cancer risk, and infant mortality rate (IMR) is an indicator of mortality. Contrast it with an index, which is obtained by combining two or more indicators.
Indirect standardisation	Removing the effect of differential structure of the group by using standard subgroup specific rates on two or more groups.
Indirectly standardised death rate	For age, same age-specific death rates are used on the observed age-structure of two or more groups to recalculate the death rate. These (called, standard) age-specific death rates are chosen by the researcher. Age is the most frequent factor for standardisation but similar standardisation can be done for any factor. Indirect standardization is used when observed specific rates are unreliable due to small numbers, or are not available.
Indrayan smoking index	A summary measure of the burden of smoking in a person taking several aspects into consideration: (i) age at start, (ii) number of cigarettes/bidi/cigars and duration, (iii) active and passive smoking, (iv) time elapsed since cessation of smoking if stopped.
Induction	The method of reaching to a generalized conclusion after compiling the individual cases
Infectiousness	The property of a disease to be able to infect susceptibles on exposure: but generally used to measure how many can be infected. Measles is a highly infectious disease since an exposed susceptible is very likely to catch the infection.
Infectivity	The proportion got infected out of those exposed.
Infographics	Representation of multiple graphs and related text into one panel.
Information bias	Suppression of some information by some subjects because of stigma or any other reason. Also loosely used for any type of bias in the data.
Informed consent	Agreement by a subject to participate in a research or some such endeavour after he is fully explained the favourable and adverse implications of participation.
Instruction bias	Use of varying individual discretion to resolve doubtful and unforeseen situations when the instructions are not complete, or not properly understood.
Instrument bias	A systematic error in an instrument to consistently give either lower or higher values than actual. Presence of air bubble in mercury column of a sphygmomanometer makes the instrument biased. Improper use of an instrument can also cause this bias.
Intention-to-treat analysis	Analysis that includes dropouts or other subjects with incomplete data or those who had to be changed from one therapy to the other due to developing a medical complication assuming worst or average scenario for them: all subjects continue to be considered in the group to which they were originally assigned.
Interaction	Simultaneous presence of two or more antecedent factors affecting the outcome either negatively or positively so that the net effect is not the same as the sum of their individual effects. It is called antagonism when the interaction effect is negative and called synergism when this is positive. But there could be other interactions that are not classifiable into any of these two categories.
Internal consistency	Mostly, consistency within the set of actual observations. One observation should be consistent with the other, or one variable should be consistent with the other. If systolic blood pressure of a person is 110 mmHg and the diastolic blood pressure is 95 mmHg then they are inconsistent unless there are specific reasons for such disparate readings. At the group level, HIV prevalence should be higher among those practicing multipartner sex. If the prevalence in this group is lower than those with single partner then this is inconsistent, and raises doubts about internal validity of data. If a study shows lower morbidity after exposure to a risk factor but lower mortality without the risk then this also may be internally inconsistent.
Internal validity (of a study)	When the biases are sufficiently under control so that the difference in the outcomes among groups can be legitimately ascribed to the hypothesized factor under investigation. Thus the results hold true at least for the subjects included in the study. An internally valid study may or may not be externally valid. There are situations when a sample provides excellent results for itself but fails when used on another sample from the same target population.
International Classification of Diseases (ICD)	A system of classification of diseases, injuries and causes of death into relevant groups, and assigning code to each condition, so as to promote uniformity and comparability across health care establishments in various countries. The ICD is revised every 10 years by the World Health Organisation to incorporate new diseases and new understandings.
Interpretation bias	Incorrect interpretation of results, either knowingly to support a particular hypothesis, or unknowingly.
Interval estimation	The process of assigning a range of values to a parameter within which it is expected to lie in repeated studies of that type.
Intervention study	A study of the impact of an intentionally introduced intervention on a predefined outcome. Experiments and trials are intervention studies.
Interviewer bias	Greater attention paid by interviewers to certain type of subjects or certain responses, relative to the others
Inter-observer variability	The variation between observers that occurs when the same measurement on the same subject is taken by different observers. A high inter-observer variability indicates poor reliability of the measurement.
Intra-class correlation	The correlation among same quantitative measurements within the same subjects or such other units at different times, by different observers, by different methods, etc.
Intra-observer variability	The variation that occurs when a measurement is taken repeatedly by the same observer. This indicates poor reliability of that observer.
Inter-rater reliability	The extent of agreement between the measurements obtained by different raters when they use the same measuring device on the same group of subjects.
Inverse probability	The conditional probability P(B/A) compared to P(A/B): often used to find predictivities P(D+/T+) and P(Dâ€“/Tâ€“) using sensitivity P(T+/D+) and specificity P(Tâ€“/Dâ€“) of a test, where D and T are for disease and test, respectively.
Item analysis	A set of techniques for assessing the efficiency of items of a test for properly discriminating between the good and not-so-good outcomes.
Kaplan-Meir method	The method of survival analysis that is used when the exact survival duration is assessed
Kappa	A measure of agreement in excess of chance in qualitative data; used for assessing inter-rater reliability and for other such agreements.
Keywords	The set of words that describes the essential features of a study. These words are used for indexing purposes so that the article is quickly retrieved for that category.
Kuskal-Wallis test	A nonparametric test for comparing central tendency in three or more groups.
Lead-time bias	Can occur when some subjects under study are enrolled in early phase of the disease and some in late phase of the disease. This may apparently show that early detected cases have higher duration of survival without any real prolongation of life.
Left-skewed distribution	See skewed distribution.
Length bias	The bias due to inclusion of disproportionately more cases with longer survival time in one group than the other: thus cases that show rapid progression of disease are not well represented.
Level of significance	The maximum tolerable probability of Type I error that is fixed in advance, such as 5%: denoted by a. In statistical terms, this is the agreed tolerated threshold of probability of rejecting a true null hypothesis.
Life expectancy	The average number of years a person is expected to live in a given community on the basis of current pattern of mortality. It can be calculated â€˜at birth' or at any other age. A life expectancy of 36 years at age 40 means that the average life span after the age 40 years is 36 years in that community. In this population, life expectancy at birth could be only 71 years. Life expectancy is a mortality indicator measured in terms of survival duration.
Life table	A summary of the death and survival pattern of a group of people
Life table method	The method of survival analysis that is used when the survival is assessed at fixed time intervals, such as weeks, months or years. The time intervals are fixed in advance.
Likelihood ratio	Relative odds of the result of interest (such as occurrence of a complication) in patients against the controls. Positive likelihood ratio measures the increase in odds of disease when the test result is positive, and negative likelihood ratio measures the decrease in odds of disease when the test result is negative.
Likert scale	A system for obtaining the response to a question by the extent of agreement with a declarative statement. For example, the statement could be â€œI am happy with the nursing services in this hospitalâ€ and the choices are strongly disagree, disagree, indifferent, agree and strongly agree.
Limits of disagreement	A procedure for measuring extent of disagreement between two quantitative measurements obtained by two methods, two laboratories, two sites, etc., on the same subjects. These limits are obtained as (mean of differences) Â± 2(SD of differences). If these limits are far too wide that can change clinical assessment, the disagreement is considered beyond clinical tolerance.
Line diagram	A diagram showing the trend by lines.
Linear regression	See linear relationship.
Linear relationship	A relationship that moves in a line with either positive or negative slope. The essential feature of a linear relationship is that one variable changes exactly by same amount when the other changes by one unit. If systolic blood pressure rises by Â½ mmHg per year of age over the entire adult age from 20 to 59 years, the relationship is linear in this age-interval. Linearity can also include many variables. See multiple linear regression, also curvilinear relationship.
Logarithmic scale	A scale that compresses large and extremely large values to relatively small values: log 10 is 1, log 100 is 2, log 1000 is 3, etc.
Logistic regression	The regression where the dependent variable is the probability of occurrence of an event. The independent variables may be qualitative or quantitative. This regression is based on a specific mathematical form, called logistic model.
Longitudinal study	A study where the same set of individuals is periodically assessed for one or more defined outcomes.
Mann-Whitney test	A nonparametric test for comparing central tendency in two groups: analogous to t-test for Gaussian data. Gives exactly same result as Wilcoxon test.
Mantel-Haenszel procedure	A statistical procedure for stratified analysis of qualitative data that combines evidence from two or more inter-related contingency tables.
Masking	Whereas the term blinding is used for the subjects and investigators, masking is for the regimen and the procedures. They are packaged or administered in a manner that they look similar.
Master chart	An arrangement of data that prepares one record for each subject: thus data available in several pages of questionnaire/ schedule are converted to one line in, say, Excel software. This helps to get full view of the data in one shot.
Matching	Deliberate selection of control subjects that have the same characteristics as the cases except for the disease or the condition under study to increase the comparability. In practice only a few characteristics can be matched. Mostly it is one-to-one matching, which is called pair-matching, but sometimes can be group matching also. In the former, each control is matched with one case, and in the latter one group is matched with the other on average or for the pattern on the whole.
Mathematical model	An equation that expresses an outcome in terms of set of antecedents. This necessarily is a simple representation of a complex process. See also model.
McNemar test	A chi-square test used for paired qualitative data, e.g., same subjects tested by histology and polymerase chain reaction (PCR) for extrapulmonary tuberculosis. McNemar test will reveal whether histology and PCR significantly disagree or not.
Mean	The average.
Measurement bias	Systematic error in measurement. This could be either due to faulty instrument, or due to carelessness of the observer.
Measures of association	The parameters that quantify the degree of association between two or more qualitative factors. Chi-square based measures are (i) phi coefficient, (ii) Cramer's V, and (iii) contingency coefficient. More useful measures are (i) proportional reduction in error, and (ii) relative risk or odds ratio. For data on ordinal scale, these are Kendall is tau, Somer's d and Goodman-Kruskal gamma.
Median	The most middle value obtained after arranging values in increasing or decreasing order. Median seeks to divide the group in two equal halves, each with n/2 individuals. Sometimes in practice exactly equal halves are not possible, and they are divided into nearly equal halves.
Mediators and moderators	Those extraneous factors that can alter the nature and extent of a relationship. Mediators are those that emerge after the cause starts to operate and moderators are those that are present before the cause starts to operate.
Medical decision process	The process of taking decisions regarding diagnosis, treating or not treating a patient, what treatment to prescribe, when to stop, etc., after considering the chances of success of various alternatives and their respective utility in terms of likely outcome.
Medical ethics	The discipline that considers individual patient's welfare above everything else
Medically significant	A result that is capable of modifying the management of any aspect of health or disease.
Medical uncertainties	Uncertainties in medical measurements and outcomes due to various sources of variation, and other factors such as lack of knowledge, poor compliance, incomplete information on the patient, etc. These can be diagnostic, treatment, prognostic predictive, or other types of uncertainties. See aleatory uncertainties, epistemic uncertainties.
MedLine	An electronic database of citations from nearly 5000 medical journals published in different languages in different parts of the world. This is the most useful resource of medical research literature, and the dominant part of PubMed.
Memory lapse	See recall bias.
MeSH	Medical Subject Heading: an important resource to search articles in MedLine. This reduces problems arising from, e.g., British and American spellings, and has a tree structure that branches off into a series of progressively narrower terms.
Meta-analysis	A procedure of combining evidence in different reports on the same aspect. If different trials on the same regimen report varying efficacy, they can be combined to come to a unified conclusion, which may command substantially more confidence than result of any one of the individual trials.
Metric scale	Measurement in terms of numerics such as blood glucose and cholesterol level. Contrast it with measurement in terms of attributes such as gender and signs-symptoms. Metric scale gives rise to quantitative data.
Mid-course bias	During the course of the study period, some patients who develop conditions unrelated to the one under investigation, such as injury, and thus have to be excluded. Some may have to be excluded because of related but serious condition requiring special care. In a field trial, this bias can occur when a new health facility or a new health problem starts in the study area that was not visualised earlier, and has potential to affect the results.
Misclassification	Classifying diseased as healthy (or nondiseased) or nondiseased as diseased. The first could be called missed diagnosis and the second as misdiagnosis. In place of healthy/diseased this could be any other categorization.
Misdiagnosis	Diagnosing a person as suffering from a particular disease when he does not have that disease (the person can have any other disease).
Missed diagnosis	Not being able to detect a particular disease in a person when it is present.
Mode	The most commonly occurring value, i.e., a value seen in highest number of subjects.
Model	A simplified version of a complex process. A model could be mathematical, graphical, structural, etc.
Moose statement	Guidelines for reporting of meta-analysis and systematic reviews of observational studies.
Morbidity	Any aberration in health
Multicentric study	A study conducted at different locations with a common protocol.
Multicolinearity	Existence of high correlation between two or more independent variables in a regression analysis setup.
Multifactorial aetiology	Occurrence of disease depending on multiple factors: hypertension is a univariate disease because the diagnosis depends entirely on blood pressure level, but it has multifactorial aetiology since its occurrence depends on heredity factors, life stress, diet, obesity, etc. Malaria has one-factor aetiology.
Multiple comparisons	Several comparisons based on the same data. If each comparison is statistically done with 0.05 level of significance, the total probability of Type I error can be enormously large. To keep this within the specified level, statistical procedures such as Tukey, Bonferroni and Dunnett are used for comparison of group means.
Multiple controls	More than one control subject for each case. In a case-control setup, sometimes it is easier to enrol controls than cases. The reliability of the results can be increased in this situation by enrolling 2 or 3 or even 4 controls per case.
Multiple correlation coefficient	The degree of linear relationship of one quantitative variable with two or more simultaneously considered quantitative variables.
Multiple linear regression	A regression in which a dependent variable is sought to be explained by linear combination of more than one independent variables: such as regression of systolic blood pressure on age, obesity, and socio-economic status. For one dependent and one independent variable, see simple linear regression.
Multiple regression	A regression that expresses the nature of relationship of one (dependent) variable on two or more of the other (independent) variables. This could be linear or nonlinear.
Multiple responses	More than one response to one question or one item, such as two or more complaints of a patient at the same time, or listing of two or more sources of infection when asked about HIV.
Multiplication rule (of probability)	The probability of joint occurrence of two or more independent events is the multiplication of their individual probabilities.
Multistage random sampling	The process of sampling where a subset is chosen at random from units at different stages. First stage units can be cities, second stage hospitals (within chosen cities), third stage wards (within chosen hospitals) and fourth stage patients (within chosen wards).
Multivariate analysis	A set of statistical procedures that considers several variables together for drawing a conclusion. If the variables are inter-related, as they would in most situations, the results of multivariate analysis could be very different from separate univariate analyses.
Multivariate diagnosis	The diagnosis that depends on a multitude of measurements. The diagnosis of liver cirrhosis depends on oedema, ascites, spider naevi on the chest, oesophageal varices, gastric ulcer, etc., whereas the diagnosis of diabetes mellitus depends only on blood glucose level. The former is a multivariate diagnosis and the latter is univariate.
Multivariate setup	A situation where several variables are considered simultaneously.
Mutually exclusive events	The set of events wherein only one can occur at a point of time. Blood group of a patient would either be O, or A, or B or AB. These are mutually exclusive. Signs-symptoms such as pain, diarrhoea and vomiting are not mutually exclusive
Natural experiments	Intervention by nature opposed to interventions by humans, earthquakes, iodine deficiency in water and climate are the examples that can modify the outcome.
Necessary cause	A cause that must be present to change the outcome. Exposure to an infection is necessary for it to produce that particular disease but it is not sufficient since in some cases infection can remain subclinical and may not produce the disease. Hypertension is not a necessary cause of stroke (neither it is sufficient). See sufficient cause.
Negative association	Presence of one factor associated with absence of the other, and vice-versa, in more subjects than expected by chance.
Negative correlation	Higher values of one variable generally accompanied by lower values of the other, and vice-versa, i.e., the two variables tend to move in quantitatively reverse direction.
Negative predictivity	Short for â€˜predictive value of a negative test'. This is the probability that a person with negative test really turns out to be free from the disease. Since a test is used only on suspected cases, the predictivity should be evaluated on the basis of suspected cases only.
Negative trial	A trial that reports that difference between the test and control regimens is not statistically significant, i.e., the test regimen is not found effective.
Nested case-control study	A case-control study where cases are identified through a prospective study. Controls may or may not be from the prospective study.
N-of-1 trial	A trial on one patient who undergoes repeated pairs of treatment periods such that he gets experimental treatment one period and the control therapy the other period. The sequence can be randomised. The patient and the physician can be blinded regarding the sequence. Treatment periods are replicated until a result one way or the other is obtained.
Nominal scale	Assessment of a characteristic in terms of names only. Blood group is on a nominal scale since O, A, B and AB are just names with no order or no grading among them. The other type of scale for qualitative variable is ordinal where grading is present such as hypotensive, normotensive, probably hypertensive and definitely hypertensive.
Nomogram	A collection of inter-related lines or curves such that the corresponding values can be read by using straight-edged ruler put across those lines or curves.
Noninferiority trial	A trial to examine if one regimen is not inferior than the standard regimen by more than clinically unimportant margin.
Nonlinear relationship	A relationship that is characterised by a curve instead of a line. Oestrogen level in a woman has a cyclic variation over menstrual periods, and thus the relationship is nonlinear. Many organisms multiply exponentially as the days after exposure pass, and not linearly.
Nonparametric test	A statistical test of hypothesis that does not focus on a parameter such as mean. This does not require Gaussian or any other specific form of distribution of the variable. The usual tests such as t and F require Gaussianity but chi-square is nonparametric. Other popular nonparametric tests are Mann-Whitney (or Wilcoxon), Kruskal-Wallis, and Friedman.
Nonrandom sample	Same as purposive sample. Can include volunteers, referred cases, case series, convenience sample, etc.
Nonrandomised control trial	A trial in which the subjects are assigned to the case group and the control group as per the convenience. Contrast it with RCT.
Nonresponse	Not being to able to collect full or partial information on subjects once they are included in a study. This can happen due to unrelated death, injury, moving out of the area, left against medical advice, refusal to cooperate, etc.
Nonsampling errors	Opposed to sampling errors, these arise mostly due to lack of planning or due to lack of knowledge. The examples are presence of confounders, nonresponse, partial compliance, biased sample, inadequate measurement, etc.
Nonsense correlation	A correlation between two variables that incidentally occurs because each is related to a third irrelevant variable. Such correlation has no biological plausibility. For example, there might be a correlation between births in India and temperature in Boston, both of which rise in the months of August and September each year.
Normal deviate	The difference of a value from its mean when expressed in SD units, such as LDL cholesterol in a patient being more than 1.25SD away from the mean in healthy subjects. Normal deviate is 1.25 in this example. Compared to the absolute difference, this deviate gives a more realistic assessment of how far the value is from mean. Generally, same as z-score.
Normal distribution	Same as Gaussian distribution.
Normalisation (of variables)	Standardisation of variable values to (0,1) scale.
Normal level	A level generally seen in healthy individuals. This is not necessarily ideal or optimal. Normal level may be different for children than for adults, or different for males than for females, etc.
Normal range	The range of values (of a quantitative medical measurement), which is generally seen in healthy individuals in a population or its specified segment.
Nuisance variable	A variable not of interest but interfering and spoiling the picture.
Null hypothesis	A hypothesis that says that there is no difference, or that asserts the existing knowledge, and is tested for refutation by the study.
Number needed to treat (NNT)	The average number of subjects that must be treated to get one favourable outcome or to prevent one adverse outcome. If improved blood pressure control of 15 subjects is required for 10 years to prevent one death from myocardial infarction, then NNT for this outcome is 15 subjects for 10 years. Mathematically, this is reciprocal of absolute risk reduction.
Observational study	A study based on observation of the natural occurrences (no intervention). See case-control study, cross-sectional study, prospective study,
Observer bias	Observer being more careful or attentive to specific type of patients or particular responses.
Odds	The chance or frequency of occurrence or presence of a characteristic relative to its nonoccurrence or absence. If the chance of occurrence is 75%, the odds are 3:1. Generally calculated for presence of antecedent factors.
Odds ratio	The ratio of odds in one group to the other (generally the control group).
One-sided alternative	A directional alternative hypothesis in the sense of asserting that the value can be only either more or less than the null value. Contrast it with the two-sided alternative.
One-tailed test	While testing equality of two groups, it is sometimes not known beforehand that which group could be better (or worse). For example, this happens when a test regimen is being compared with the existing regimen. This requires a two-tailed test. However, while comparing a test regimen with placebo, if there is an assurance that test regimen can not be worse than placebo, one-tailed test is used.
One-to-one matching	See matching.
One-way classification	The division of subjects of interest by only one characteristic, such as dividing cases of bronchial asthma by their smoking status only.
One-way design	A study that is planned to investigate the effect of levels of only one factor. The levels could be two such as presence and absence, or more than two such as none, mild, moderate, and serious.
Open-ended question	A question whose answer is allowed to be recorded in verbatim as given by the respondent. Contrast it with a close-ended question that provides a list of possible answers.
Open trial	Nonblind trial.
Ordinal association	Association between two ordinal characteristics such as severity of disease and socio-economic status. This is measured by Kendall's tau, Somer's d or Goodman-Kruskal gamma.
Ordinal scale	A scale that measures a polytomous characteristic in a defined order, such as severity of disease into mild, moderate, serious and critical. The â€˜distance' between mild and moderate is undefined. Division of blood group into O, A, B, and AB is polytomous but not ordinal since these blood groups do not have any order
Outcome	A disease or a health condition of interest including any change in health status that may occur after exposure to antecedents or interventions. It may or may not be a result of the antecedents.
Outlier	A value that is far away from the other values. If duration of hospital stay after a cholecystectomy is 2, 3 or 4 days for most patients but happens to be 14 days for one patient because of complications, this value 14 days is an outlier.
Outlier bias	Differential management of different outliers: e.g., considering some extreme value as outlier and not others, or not ignoring outliers if they support a particular hypothesis.
Over matching	Matching for confounders that affect both the outcome as well as the risk factors. Such matching can produce biased results.
Pair-matching	See matching.
Paradigm	A system, a pattern of thought, or a model regarding a phenomenon.
Parallel control group	Opposed to treatment group that receives regimen under test, parallel control group is another group of subjects that receives either placebo or an existing regimen. Contrast it with before-after study where there is no separate (parallel) control group.
Parameter	A summary measure for any characteristic in the target population, such as percentage of cirrhosis patients with high aspartate aminotransferase, or rate of increase of systolic blood pressure in healthy subjects per year of age. The parameter pertains to the entire population of interest and not to the sample.
Parsimonious model	A model containing small number of factors yet providing adequate explanation.
Partial correlation	The correlation between two quantitative measurements when third or other measurements affecting them are considered fixed, and thus their effect is eliminated. Generally considers only the linear relationship.
Pathogenecity	The ability to produce the clinical manifestation of disease in an infected person. Measles is not only infectious but also highly pathogenic
Pearsonian correlation coefficient	A measure of the degree of linear relationship between two quantitative variables. This ranges from â€“1 to +1 with zero in between indicating no linear relationship. A negative correlation means that increase in one is accompanied by decrease in the other or vice-versa, whereas a positive correlation means that both increase or decrease together at least to some extent.
Peer review	A refereeing process of a research proposal, article, thesis, etc., by expert colleagues for technical merit.
Percentiles	Ninetynine cut-points of a variable that divide a group of subjects into one hundred segments after arranging in ascending order such that each segment has the same number (n/100) of subjects.
Person-years	The sum total of years observed for different individuals. If one person is followed-up for 3 years, second for 1Â½ years and third for 2 years, then the person-years of follow-up is 3 + 1Â½ + 2 = 6Â½ years. Similarly there could be person-weeks or person-months.
Phases of a trial	In phase I, the maximum tolerated dose and pharmacological properties including toxicity and safety are determined by a trial on a group of volunteers, usually without controls. The objectives of phase II are to investigate clinical efficacy, incidence of side-effects, identify a dose schedule, and to collect further pharmacological data. Most phase II trials have a control group. Phase III is an RCT that is done after achieving success in the first two phases to firmly establish efficacy and safety. Post-marketing surveillance is sometimes called phase IV.
PICO method	A method to divide the research question into its components so that nothing is missed. P: problem, population, patients; I: intervention or exposure, indicator; C: control comparator; O: outcome.
Pie diagram	A diagram showing the proportions of subjects in different groups or with different mutually exclusive characteristics by means of segments of a circular pie.
Pilot study	A small-scale forerunner study to learn about the situation and the variables.
Placebo	An inert substance or a procedure that is neither harmful nor beneficial. The objective of placebo is that the subject gets the perception that he is receiving treatment
Placebo-controlled trial	Study of efficacy and safety of a regimen in comparison to a group that receives placebo. Contrast it with trials in which the comparison group receives existing (active) regimen.
Placebo effect	The psychological effect on a patient of the perception that he is receiving a treatment although the treatment is dummy. This is the main reason for conducting placebo-controlled trials. Also see Hawthorne effect.
Plagiarism	Unauthorized use of someone else's language and thoughts and projecting them as your own.
Point estimation	The process of identifying a single value of a parameter as an estimate based on the study group.
Polytomous variable	A characteristics divided into three or more exclusive categories, such as severity of disease into mild, moderate, serious and critical, or liver disease as cirrhosis, hepatitis, and malignancy. A quantitative measurement such as cholesterol level can be made polytomous when divided into small number of categories such as -99, 100-179, 180-249 and 250+ mg/dl.
Population	The totality of individuals or units of interest. There could be a â€˜population' of blood samples collected in a year. If the interest is restricted to only suspected cases of liver diseases, the population comprises blood samples of such cases only. If the interest is further restricted to the cases attending OPD in a group of hospitals, the population is also accordingly restricted.
Population attributable risk	The risk in the total population minus the risk in unexposed subjects. In the population, some are exposed but most are unexposed. This measures the impact on the population of eliminating that exposure.
Positive association	Presence of one factor associated with presence of the other, and absence with absence, in more subjects than expected by chance.
Positive correlation	Higher values of one variable generally accompanied by higher values of the other and lower values with lower, i.e., they tend to move in the same direction.
Positive predictivity	Short for â€˜predictive value of a positive test'. This is the probability that a person with positive test really turns out to be suffering from the disease. Depends heavily on the prevalence of the disease. Since a test is used only on suspected cases, the predictivity should be evaluated on the basis of suspected cases only.
Posterior probability	See prior probability.
Posthoc comparison	The comparison of groups with regard to their initial equivalence after collection of data.
Post-marketing surveillance	Keeping a tab on outcome and side-effects of a formulation after it is introduced into the market: sometimes called phase IV of a clinical trial.
Post-test probability	The probability of occurrence or presence of an event such as disease after the test results are available. See prior probability.
Power	The probability that a study or a trial will be able to detect a specified difference. This is calculated as 1 â€“ Probability of Type II error
PowerPoint	A software of Microsoft Corporation that helps to make slides, which can be directly projected from the electronic format. This software has several features regarding designing the slide. The presentation of research to an audience can be very effective with the help of PowerPoint slides.
Pragmatic trial	A trial done under standard clinical practice so that accepted variation such as during drug intake are allowed. Contrast it with explanatory trial done under near-ideal conditions with practically no deviation.
Precision	Same as reliability but measured statistically as inverse of the variance.
Prediction error	The difference between an observed value and a predicted value based on models or based on other considerations.
Predictive model	A model that predicts the outcome on the basics of antecedents. A predictive model focuses on correct prediction and does not worry about the biological process or relevance of the predictors. Choice of predictors is not important. Two different models can be equally good in prediction.
Predictive validity (of a test)	The average of the positive predictivity and negative predictivity of a test. This can be used as a combined measure of the two types of predictivities when both are equally important for the outcome of interest.
Predictivity (of a test)	See positive predictivity and negative predictivity.
Prevention trial	A human experiment for a preventive strategy such as exercise and diet changes or a regimen involving vitamins, to prevent occurrence or recurrence of a disease, or any other adverse condition.
Pretest probability	The probability of occurrence or presence of an event such as disease before the test results are available: generally the same as prevalence rate in the specified group. Contrast it with post-test probability. See also prior probability.
Pretesting	Checking the workability, adequacy, reliability, etc., of a tool before using it for actual study. The tool could be an instrument, a laboratory procedure, a questionnaire, or any other.
Prevalence	The number of cases of interest present or existing at any specific time, usually at the time of the survey.
Prevalence rate	Prevalence per unit of population or per unit of susceptibles, such as percent, per thousand and per million. Note that prevalence rate is not a â€˜rate' as it does not signify frequency of occurrence per unit of time
Prevalence ratio	Ratio of prevalence rate in one group to the other.
Prevalent cases	Same as prevalence.
Primary data	Data that are directly collected from the respondents. Contrast it with secondary data that already exist in databases, records, reports, articles, etc.
Primordial factors	Factors that work behind the scene, are precursors, or are those that give rise to risk factors. Life style is a primordial factor that can give rise to risk factors such as obesity and smoking.
Prior probability	The chance of occurrence or presence of an event such as disease or death in a patient when nothing is known about the condition of the patient. Once something such as signs-symptoms-measurements are known, the diagnosis becomes substantially more focused and the probability changes. The latter is called the posterior probability. When further information becomes available, this posterior becomes prior probability and the new probability based on the fresh information becomes posterior.
PRISMA	Acronym for Preferred Reporting Items for Systematic Reviews and Meta Analyses: A checklist of 27 items and a flow diagram of how the studies for reviews were selected and processed for joint conclusion.
Probability	A measure of belief in occurrence of an event or presence of a characteristic. This can be obtained either on the basis of theoretical considerations such as 1/6 for each of the 6 faces of a dice, on the basis of experience, or on the basis of frequency of occurrence when total occurrences are very large. Probability is the degree of certainty of occurrence of an event on a 0 to 1 scale. Probability of death is 1 for all individuals but the probability of death of a pancreatic cancer case within 5 years of detection could be 0.6.
ProCite	The software that manages bibliographic citations; can be used in conjunction with MedLine.
Proforma	A prototype or a sample of a format on which the observations are to be recorded.
Prognostic factor	A characteristic that can help in prediction of the eventual development of an outcome such as recovery, complication, and death. This does not necessarily imply a cause-effect relationship.
Prognostic stratification	A stratification done after examining the pattern of observations, such as categorising patients as mild, moderate, serious on the basis of new criteria developed after the patients are seen. In the usual stratification, the criteria are decided before seeing the patients.
Prophylactic trial	An experiment on a prophylactic measure such as amnioinfusion for meconium-stained amniotic fluid at the time of childbirth, or in the community such as iron supplementation to adolescent girls.
Proportion	The measure of how big is the part of a whole. The whole is considered as one. If 20% of a population have blood group A, the proportion is 0.20 or 1/5.
Proportional hazards model	A model that works when ratio of logarithm of hazard in test group to control group remains same all through the period of observation. This is an important prerequisite for the validity of Cox model.
Proportional reduction in error	The reduction in error in prediction of the outcome when a particular antecedent is used for prediction relative to when it is not used.
Prospective study	A study that investigates outcomes for known antecedents. The follow-up of subjects is inherent in this kind of study since the occurrence of outcome can take time.
Protocol	A comprehensive statement regarding steps to be taken
Proximal measures	Measurements directly on outcome, contrasted with distal measures. Impact of vitamin A supplementation to children below three years can be measured proximally by rise in retinol level. In contrast, distally, the impact can be measured by growth pattern. Research results many times depend on appropriate choice of proximal measures for the outcome of interest.
PubMed	An extension of MedLine database of articles published in selected journals.
Publication bias	Publication of one type of articles more often than the other types, such as more frequent publication of positive results than negative results.
Purposive sampling	Nonrandom sampling to include subjects that serve the specific purpose, such as volunteers in phase I of clinical trials. See convenience sampling, haphazard sampling, snowball sampling, volunteer studies.
Putative factors	The risk factors popularly believed to be the causes of an outcome â€“ they may not be the actual operational factors.
P-value	The probability of Type I error, i.e., the chance that a difference or association is concluded when actually there is none: the chance that the result could have been produced by random sampling fluctuations rather than being actual.
Qualitative data	A set of observations on qualitative characteristics of individuals such as signs and symptoms. These can be nominal or ordinal. The only real summary measure for qualitative data is proportion of subjects with a specified characteristic, although for some ordinal data, scores can be assigned that can be treated as numerics.
Qualitative variable	A characteristic that is assessed in terms of attributes such as gender and degree of severity of disease: a variable that yields qualitative data.
Quality of life	Comfort and functionality of persons, generally as perceived by the person himself.
Quality of life trial	A trial for a regimen that could improve quality of life of people in ill-health, particularly those with chronic diseases or degenerative conditions.
Quantiles	Cut-points of values of a variable that divide the total number of subjects into desired number of equal groups. Examples are percentiles, deciles, quartiles and tertiles.
Quantitative data	Collection of observations on characteristics that could be numerically expressed for an individual such as haemoglobin level, blood pressure, and blood glucose level. Most common summary measures for quantitative data are mean and standard deviation (SD).
Quantitative variable	A characteristic that is measured in terms of numerics, such as creatinine level and parity of a woman: a variable that yields quantitative data. See also continuous variable, discrete variable.
Quartiles	Three cut-points of values of a variable that divide a group in four equal parts with regard to number of subjects, after the values are arranged in ascending order.
Quasi-random allocation	Allocation of subjects to test and control group by following apparently random method such as alternation and based on birth data that are not strictly random.
Questionnaire	A survey instrument that contains a predetermined series of questions that are supposed to be put in verbatim to the respondents or can be self-administered. It contains space for recording responses also.
Quota sampling	Purposive selection of pre-specified number (quota) of subjects from each segment of population without using random method.
Random	Unpredictable, like lottery.
Random allocation	Same as randomisation.
Random effects model	A statistical model that stipulates that the levels of factors under study are random samples of the possible levels. Contrast it with fixed effects model.
Random error	An error that has no bias, and which is natural to occur in observations because of biological or other variation beyond control. These errors are small, and some are positive some negative so that the long-term average is close to zero. Most random errors are small and few are large. This gives rise to Gaussian distribution of such errors.
Random sampling	Sampling in a manner that the selection can not be predicted. The chance of selection of various units can be equal or unequal. The popular methods of random sampling are simple, systematic, stratified, cluster, and multistage.
Randomisation	Allocation of subjects to different groups in a random manner with equal chance. The objective is that unaccounted factors are almost equally distributed among groups, and there is no bias on this count. Randomisation could be open so that the participants or the observers know which subject is in which group, or it could be concealed.
Randomised clinical trial	An experiment on human beings where the subjects are randomly allocated to various arms of a trial. These arms may be various dosage groups. When one arm is the control group, this becomes randomised controlled trial.
Randomised controlled trial	A trial where there is a control group (in addition to the test group) and the allocation of subjects to the control and test groups is by random method. This is considered to be the ideal methodology to evaluate efficacy of a new regimen (preventive, therapeutic or diagnostic) particularly when it is double-blind.
Rasch analysis	The process of recalibration of a test (such as a questionnaire) so as to increase the chance of higher correct answers of more difficult questions by more able persons.
Rate	The frequency with which events occur per unit of time, such as deaths per year or new cases per month. Time is a necessary ingredient of a rate. Generally measured per unit of population such as percent, per thousand and per million.
Ratio	Strength or magnitude or number of one quantity relative to the other, such as male-female ratio and albumin-globulin ratio.
RCT	Short for randomised controlled trial.
Recall bias	Not being able to recall events occurring far away in the past with the same frequency as those occurring recently. This introduces bias in favour of recent occurrences. Also occurs when diseased cases are able to recall because of their suffering but controls fail to recall as much. Also when serious episodes are easily recalled and mild episodes tend to be neglected.
Receiving operating characteristic curve	Same as ROC curve.
Record linkage	The process of linking different records of one person to make one comprehensive record.
Reference population	Same as target population.
Reference values	Same as normal levels.
Referred sample	A group of subjects that are referred for specialised handling. A study can be carried out on a referred sample although the results would be biased.
Regression analysis	The statistical procedure to find a regression equation.
Regression equation	The nature of relationship of one variable with one or more of others, generally expressed as a mathematical equation that best fits the data. Also called regression model.
Regression coefficient	The quantity that delineates the change in dependent variable for one unit change in independent variable. If regression coefficient of birthweight (in gm) on maternal haemoglobin level is +60, it means that birthweight increases on average by 60 gm for each 1g/dl increase in maternal haemoglobin level.
Regression line	Graphical presentation of linear regression.
Regression model	Same as regression equation.
Regressor	Same as independent variable in a regression equation.
Relationship	The property of change in one variable when the other changes. This change can be causal, incidental, or due to a third intervening variable.
Relative risk	Risk of occurrence of an outcome in the presence of one factor (exposure) relative to the risk in the presence of another (generally control) factor.
Relative risk reduction	Reduction in absolute risk after an intervention as percentage of the risk in exposed group before intervention.
Reliability	Ability to repeat the performance. The performance could be poor but same performance every time means good reliability. Statistically, this means smaller variance in repeated measurements.
Repeatability	Same as reproducibility.
Repeated measures	When the same subject is observed repeatedly after specified time gaps, such as monitoring blood pressure and heart rate at 1, 5, 10, 15 and 30 minutes after a administering anaesthesia.
ReplicationÂ	Trying the same regimen on more than one equivalent group so as to get an idea of the repeatability. Replications, when yielding similar results, increase the reliability of the results.
Reporting bias	Highlighting findings in a report that support a particular view at the cost of the other.
Reproducibility	The ability to give similar result when conducted in identical conditions.
Reproductive rate (of infection)	The rate at which an index case infects others in the entire transmission phase. If the reproductive rate is one or more, the infection sustains itself and spreads in the population, like HIV is doing in some countries. If the reproductive rate is less than one, the infection will die down or will stabilize at a low level.
Research	Discovery of new facts, enunciation of new principles, or fresh interpretation of the known facts or principles.
Research design	Same as design.
Research protocol	Statement on planned steps of research, including background information and rationale, objectives and hypotheses, review of literature, methodology, ethics, statistical evaluation, and references.
Response bias	(i) Not giving proper history due to stigma such as in STDs or for any other reason, and (ii) selective nonresponse, i.e., persons who are not seriously ill do not fully cooperate.
Retrospective cohort	Same as historical cohort.
Retrospective follow-up study	A study based on a historical cohort.
Retrospective study	A study that investigates antecedents for known outcomes. The recruitment of cases can be prospective spanning a duration such as all cases reporting in one year period. But the logic is from effect to the cause.
Reverse causation	Also called retro causation or backward causation, reverse causation refers to a situation where apparently the outcome precedes the cause. For example, the studies might indicate that obese people are more depressed while actually depression may change diet preferences and cause obesity.
Right-skewed distribution	See skewed distribution.
Risk	The chance of occurrence of an outcome of interest, generally calculated per year of exposure. It is not necessary that the outcome is adverse to health. See attributable risk, population attributable risk, and relative risk.
Risk difference	The difference in risk of occurrence of a condition in two setups, such as risk of glaucoma in vegetarians vs. risk in nonvegetarians. Same as attributable risk.
Risk factor	A characteristic that is suspected to affect the outcome, such as obesity for hypertension. Risk factor could be an aspect of personal behaviour or life-style, an environmental exposure, an inherited or in-born characteristic, or any other.
Risk ratio	Same as relative risk.
Robust method	A method that is not much affected by minor variation in its applicability conditions.
ROC curve	A curve that depicts the relationship between sensitivity of a test for different thresholds and the corresponding (1â€“specificity), such as for different T4 values for diagnosis of hyperthyroidism. This curve helps to evaluate the applicability of a test and helps to compare performance of one test with the other, such as of T4 with T3, or T4 with TSH.
Sample	A part of the target population, which is actually studied.
Sample size	The number of subjects or units in a sample.
Sample survey	A cross-sectional study of antecedents and outcomes on a sample of subjects from a defined population. Generally, this is a large-scale survey covering a full slate or full country.
Sampling	Choosing a part from the whole, such as choosing 300 child births out of 5000 in a hospital in one year for studying the intrauterine growth retardation. See random sampling, purposive sampling.
Sampling bias	The bias due to (i) nonrandom sample such as volunteers that do not represent the target population, and (ii) small sample that fails to represent the entire spectrum of subjects.
Sampling design	The method of selection of sample out of the population of subjects.
Sampling error	The tendency of one sample giving a different result than the other sample, and neither possibly giving exactly same result as in the population. This is not an error in conventional sense but is natural fluctuation.
Sampling fluctuation	The tendency of different samples giving different results, even if drawn from the same target population. This happens because different samples contain individuals who are different from those in other samples.
Sampling fraction	The extent of sampling such as one out of eight, or one out of twenty.
Sampling frame	A list of units in the target population from which sample is drawn.
Sampling method	The procedure to choose or select a fraction out of the target population.
Sampling unit	The unit used for sampling. In a multistage sampling, there is a separate sampling unit for each stage.
Sampling variation	Same as sampling fluctuation.
Scales of measurement	The system of differentiating one type of observation from the other. When names are used (opposed to grades) for differentiation, the scale is called nominal. Signs and symptoms are generally measured on nominal scale. Textual grades such as mild, moderate, serious, are measurements on ordinal scale. Numeric measurements such as body mass index are on metric scale.
Scatter diagram	A diagram displaying values of one quantitative variable for different values of the other variable by plotting points. This is also called the (x, y) plot.
Schedule	A form that contains a set of items on which information is to be obtained. It contains space also to record the responses.
SciSearch	The software of the Institute for Scientific Information for searching citations in Science Citation Index and Current Contents.
Score	Quantification of a set of (mostly) qualitative measurements, such as APACHE score. Scores in medicine are used either to grade severity of a condition, as an aid to reach to a diagnosis, or to assess prognosis.
Screening test	A criterion used to locate possible positives that can be later confirmed by stricter criterion. A screening criterion should have high negative predictivity. The criterion could be a laboratory test, a radiological test, or a clinical observation.
Screening trial	A trial for evaluating efficacy and safety of a procedure that can detect the disease or can detect risk factors.
SD	Short for standard deviation.
Secondary attack rate	The number of people who get sick within the incubation period after exposure to an infective person. This infective person is called the index case.
Secondary data	The data that are already lying somewhere as in records or literature. Contrast it with primary data.
Selection bias	The bias occurring due to selection of nonrepresentative group of subjects
Sensitivity	Ability to identify known positives as positives. If CPK Â³120 IU is present in 60% known cases of myocardial infarction, then sensitivity of this cut-off is 60%. Sensitivity is not a valid indicator of diagnostic value of the test. It only measures the intrinsic ability of a test to detect a disease when it is already known to be present.
Sensitivity analysis	The analysis to assess the impact of changes in assumptions on the outcome. Because of limitation of knowledge, various assumptions are made in a study. For example, it is generally assumed that cure rate depends linearly on treatment regimen, host characteristics and compliance. Two kinds of epistemic uncertainties arise
Sequential sampling	Serial sampling of subjects, one by one, to be stopped when scientifically acceptable result either way is available.
Severity score	A score used to grade severity of a condition such as Apgar score and APACHE score.
Sign test	A nonparametric test based on sign (negative or positive) of differences between two sets of observations. This test is used to compare two groups for their central tendency.
Significance	See statistical significance and medical significance.
Significance level	Same as level of significance.
Simple linear regression	A regression in which a dependent variable is sought to be explained linearly by only one independent variable, such as regression of systolic blood pressure on age. Other correlates are ignored in this setup, and the relationship format is limited to linear.
Simple random sampling	Sampling in a manner that all individuals have same chance of being selected.
Simple regression	Relationship between one independent and one dependent variable. It could be linear or nonlinear.
Simpson's paradox	In some cases the tendency of aggregated data showing a result very different from the results from disaggregated (stratified) data. This can happen due to interaction effect of the stratifying variable.
Simulation	Generation of data based on certain model.
Single blind	No subject is informed that he is receiving test regimen or the control regimen in a trial. But the investigator is aware of the allocation.
Skewed distribution	Opposed to symmetric such as Gaussian, generally a distribution is considered skewed when values on one side of mode vary much more than on the other side. It is right-skewed when values more than mode have more variation, and left-skewed when values less than mode have more variation.
SMART objectives	The desirable format of the objectives of research: Specific, Measurable, Achievable, Relevant, and Timely.
Smoking index	A measure of life-long burden of smoking with weightage for age at initiation, quantity of smoking, duration of smoking, type of smoking (filter/ nonfilter/ bidi/ cigar), and duration elapsed since quitting by exsmokers.
Snowball sampling	One eligible person, such as a client of sex-worker, is asked to list others known to him and eligible that can be included in the sample, and those in turn are asked to identify others, so on.
Spearman's correlation	A measure of strength of relationship between two quantitative variables when they are converted to ranks.
Specificity	Ability to identify known negatives as negatives. If AFB is negative in 95% of the healthy subjects, its specificity is 95%. In practice, a test is rarely used on healthy subjects. It is used on suspected cases. In them, the specificity may be very different.
Spectrum of disease	The distribution of subjects by affected and not affected, and among affected by severity of affliction.
Spurious association	A false association that could arise due to presence of confounders, bias, or merely a chance.
Spurious correlation	A false correlation that could arise due to presence of confounders, bias, or merely a chance.
Standard deviation (SD)	Most common and generally most appropriate measure of dispersion obtained as positive square root of variance.
Standard error (SE)	The measure for sample-to-sample variability in a summary measure such as mean and median. Just as the measurements such as haemoglobin level differ from person to person, so do the sample summaries from sample to sample. Mean haemoglobin level in one sample would be different from another sample even when both are drawn from the same target population. The extent of variability in such summaries is measured by their respective standard errors. Actually, this is the standard deviation of mean or median or any other summary measure as the case may be.
Standardisation (of groups)	The procedure that makes two different groups comparable by bringing them on to a common base or a common standard, such as age-standardization.
Standardisation (of variables)	Subtracting mean from the variable and dividing by the standard deviation (SD): thus standardisation makes mean = 0 and SD = 1.
Standardised death rate (directly standardised)	Recalculated death rate when one or more factors affecting deaths (such as age structure) are brought at par with the comparison group, or both brought to a common base. This substantially increases comparability that crude death rate lacks when one group is young and the other is old. Common base structure (called, standard) of population is chosen by the researcher.
Standardised mortality ratio	Ratio of actually observed deaths to expected deaths based on standard death rates. If this ratio is more than one, the force of mortality is higher in the study group compared to the standard group.
Standard treatment	An approved or widely accepted treatment modality for an ailment.
STARD statement	Acronym for standard for Reporting of Diagnostic accuracy studies: guidelines for reporting of results of studies on diagnostic validity of medical tools.
Statistic	A summary measure for any characteristic in the sample or the group actually studied, such as mean, median or standard deviation of a sample, or proportion of subjects found affected in a sample.
Statistical analysis	Subjecting data to the rigours of statistical methods so that the uncertainty levels are either quantified or minimized, or both.
Statistical fallacies	Many fallacies can occur in the results because of improper use of statistical methods. Some of these are due to (i) use of improper denominator for computing rate or percentage, (ii) not accounting for variable periods of exposure that could affect the rate of outcome, (iii) considering mixture of two groups as one, (iv) misuse of percentages such as 2 out 4 being stated as 50%, (v) using means for emphasising a point without considering the standard deviation, (vi) inappropriate scales in the graphs, (vii) looking at linearity when in fact the relationship is nonlinear, (viii) ignoring important prerequisites such as randomness, independence, equality of variances and Gaussian form of distribution, (ix) using means where proportions are adequate, or vice-versa, (x) ignoring baseline values, (xi) using too many statistical tests on the same set of data without adjusting P-values (xii) quantitative analysis of codes, (xiii) jumping to cause-effect relationship without sufficient examination of data, and (xiv) multivariate conclusions on the basis of several univariate analyses.
Statistical power	Same as power.
Statistical significance	A result is statistically significant if the chance of wrongly rejecting null hypothesis is less than the prefixed level such as 5%. The implication is that chance differences in samples would produce that kind of result in less than 5 times out of 100
Statistical test	A procedure to find P-value corresponding to a null hypothesis on the basis of the given data. Depending upon the type of data and the type of hypothesis, a large number of statistical tests are available. Most popular of these are Student's t-test and chi-square test.
Statistical threshold (of normal range)	Mean Â± 2SD where mean and standard deviation (SD) are obtained from measurements in a large number of healthy individuals. There is a chance of classifying nearly 5% healthy individuals with extreme values as sick when this range is used as normal.
Statistics	A science that helps to manage uncertainties in the data. Also, plural of statistic.
Stopping rules	Rules for stopping a clinical trial while it is still continuing. Stopping can be done either for futility when it is found on interim assessment that there is practically no chance of concluding the desired efficacy, or can be stopped for efficacy when the interim data analysis finds convincing evidence of the desired efficacy.
Strata	The divisions obtained after stratification (the singular is stratum).
Stratification	Division of subjects into relevant groups.
Stratified analysis	A statistical procedure to adjust for the effect of confounders or other correlates without estimating their individual effects.
Stratified random sampling	Separate simple random sampling from each group after stratification.
Strength of relationship	The consistency of change in one factor when the other changes. If one unit change in one factor results in nearly the same change in the other in all the subjects, the strength is high. Note that the strength is not related to the magnitude of change. Small change, if consistent, would mean high degree of relationship.
Strobe	Acronym for STrengthening the Reporting of OBservational studies in Epidemiology: A checklist of items for reporting results of an observational study.
Structured questionnaire	A questionnaire where question have list of possible answers. The respondent has just to tick the correct response for himself.
Student's t-test	Same as t-test.
Study design	Same as design.
Study setting	The environment in which the study is conducted. The setting could be a general hospital, a referral centre, private practice, ambulatory care, community, etc.
Sufficient cause	A cause that by itself is sufficient to change the outcome. Exposure to measles virus in a susceptible is sufficient to produce the disease. In fact this cause is both necessary and sufficient. Hanging by rope is sufficient to cause death but is not necessary as a cause for death.
Superiority trial	A clinical trial that has objective to examine if the regimen under test is better by at least a predefined clinically important margin.
Surrogate outcome	Outcomes that are not of direct interest but reflect the outcome of direct interest, such as pallor for nutritional level. Surrogates are generally easy to assess and do not require long follow-up.
Surrogate variable	A makeshift substitute variable that is used when the actually required variable can not be measured. If the respondents are shy of revealing income, surrogates such as size of house, car, telephone and television can be used to assess the level of income. Since they are surrogate, they may or may not reveal the true status.
Survey	A descriptive study done generally on scientifically selected subjects. Another descriptive study methodology is case series.
Survival analysis	Analysis of survival durations. Survival duration is generic
Survival curve	A graph that depicts survival pattern of the subjects, over the observed period of time. It begins at 100% (all alive) and shows gradual or rapid decline as the time passes depending on the disease under study.
Survival function	A mathematical expression for survival curve.
Susceptibility	Proneness to catch an infection or a disease. A person who is effectively immunised against tetanus is not susceptible to tetanus, and a child is not susceptible to the usual sexually transmitted diseases.
Synergism	A situation where two factors when present together accentuate the outcome more than their individual capacities. Iron and folic acid are synergistic for increasing haemoglobin level.
Synthesis (of research)	The process of combining diverse evidence from different researches to come to a holistic conclusion.
Systematic error	Same as bias.
Systematic random sampling	Selection of first individual at random and others automatically at regular spacing depending on sampling fraction.
Target population	Same as population. Also called reference population.
Tertiles	Two cut-points of values of a variable that divide a group into three equal parts (each part with same number of subjects) after arranging in ascending order.
Test group	The group of subjects that is receiving or has received the regimen under test.
Test of hypothesis	The procedure used to test whether or not sufficient evidence exists against a null hypothesis: mostly a statistical procedure.
Test of significance	A statistical procedure to test whether or not the observations fall into a specified pattern, such as equal means of two or more groups, or following a linear trend. If they do not, the result is called statistically significant. This requires prior fixing of the level of significance that specifies the maximum tolerable probability of Type I error.
Test-retest reliability	Agreement between responses when the same instrument such as a questionnaire is administered to the same set of people again; stability of the responses in repeated use of the instrument.
Therapeutic equivalence	Comparable safety and efficacy of two or more treatment modalities when administered under the conditions specified for each modality. These conditions could be different for different modalities. Also see equivalence.
Therapeutic trial	A clinical trial on a therapeutic agent or regimen to evaluate its efficacy and safety.
Thesis	A proposal or hypothesis forwarded after a careful investigation accompanied by full details: generally the written work submitted by a candidate in fulfillment of partial requirement for the award of Master's degree.
Time-line	A chart or a table that states the durations and dates of beginning and ending various phases of a project, some of which can overlap.
Training sample	The sample of subjects which is used to develop a model. This model is subsequently tested on validation sample.
Transformation	When the data pattern is far from causation, transformation is applied such as log and inverse (1/x) so that the values tend to follow Gaussian pattern. Transformation is also done to achieve homoscedasticity.
Translational research	The research that tries to bridge the gap between discoveries and their application to actual life situations.
Transmissibility	Used in two senses: 1. The ability to transmit (infection or disease) from one to the other. HIV is transmissible through blood transfusion. Injury is not transmissible. 2. The force of transmission: when exposed what percentage of susceptibles get the infection or disease. HIV is more transmissible from male to female than from female to male.
Trial	An experiment on human subjects. See clinical trial, field trial, prophylactic trial, therapeutic trial and vaccine trial.
T-Score	Statistically, T-score of x = (x-x̄)/s where x̄ and s are the sample mean and sample SD respectively of x but medically x̄ is replaced by the optimal value so that T-score tells how far away is a value from optimal in SD units. The latter is quite commonly used for assessing bone mineral density.
t-test	A statistical procedure to test hypothesis of equality of two means, and for certain other hypotheses relating to parameters such as regression coefficient.
Tukey's test	A statistical test for pairwise comparisons of means when there are three or more groups. This test keeps the probability of the total Type I error within the specified level of significance, but this level could actually be lower than specified.
Twin studies	Studies of twins, particularly identical twins, to find the effect of differential environment on their health since genetic makeup of both is the same.
Two-sided alternative	An alternative hypothesis that stipulates that the parameter value can be higher or lower than the null value. Both directions are admissible. This type of alternative is setup when it is not known that the value is going to be higher or lower than the specified value.
Two-tailed test	See One-tailed test.
Two-way classification	Division of subjects by two characteristics simultaneously, such as dividing cases of diabetes mellitus by their obesity category and smoking category.
Two-way design	A study that is planned to investigate the effect of levels of two antecedent factors, such as effect of obesity (BMI<20.0, 20.0-24.9, 25.0-29.9 or 30.0+) and smoking (none, mild, moderate or heavy) on blood glucose level. Their interaction can also be investigated in this kind of design.
Type I error	The error of rejecting a true null hypothesis, i.e., concluding that there is a difference when actually there is none. The sample or data might be such that this lead to such a wrong conclusion. This error leads to false positive result.
Type II error	The error of wrongly concluding that there is no difference when actually some difference is present. This error leads to false negative result.
Type III error	This occurs when the researchers investigates wrong direction of effect. For example, if the population value of the parameter under test is actually more than the null value, the sample might give a value so much below that the researcher rejects the null and concludes that the population value is also below the null value.
Unbalanced design	A design with unequal number of subjects in different groups.
Uncertainty analysis	The analysis that delineates the effect of change in the value of the parameter under assessment on the conclusion. One component of this is the sampling fluctuation in the estimate of the parameters, and the other is the plausible change in the values themselves that can affect the outcome.
Uncertainty principle (in a clinical trial)	The principle that says that the outcome of various arms of trial should be a-priori uncertain. One specific uncertainty situation is that the a-priori chances of each regimen being successful are nearly equal
Unit of study	The unit (individual, patient, blood sample, biopsy, etc.) that is used to obtain the required information.
Univariate analysis	The analysis regarding one variable at a time. The combined result of several univariate analyses may be very different from the result obtained by simultaneous consideration of these variables. See also multivariate analysis.
Universe	The broad group of subjects for which the findings could be generalised or implicated. Statistically this is broader than the target population and can include future subjects.
Up-and-down trial	A trial where a start is made with an assumed average dose and it is increased or decreased by predetermined step in subsequent subjects depending upon the dose is not effective or effective, respectively.
Vaccine trial	An experiment on human population to evaluate potency, efficacy and safety of a vaccine.
Validation sample	A sample used to validate the results of the study. It could be a subsample of the original sample that is kept aside for this purpose, or could be a new sample of subjects.
Validity (of a measurement)	Ability to hit the target (or around it): ability to assess what is really intended to be assessed. Weight by itself is not a valid indicator of obesity in adults but body mass index that uses height also is. For a medical test, validity is measured by sensitivity, specificity and predictivities (positive and negative).
Validity (of a study)	The ability of a study to provide correct conclusion, considering the representativeness and size of sample, validity of measurements, and the soundness of methods. See also internal validity and external validity.
Validity (of a survey instrument or a test)	Ability of an instrument (such as a questionnaire or a schedule) or of a test to provide the information that matches with the objectives of the study. For its varieties, see concurrent validity, content validity, construct validity, criterion validity, face validity, and predictive validity.
Vancouver style (or format)	A style of writing research papers agreed by editors of more than 500 medical journals published around the world. It includes instructions on who could and should be authors, what help must be acknowledged, how to structure the text, and how to cite references. The system of citing references matches closely with the system followed by MedLine.
Variable	A characteristic that varies from person to person, or from situation to situation. Platelet count in different persons is variable but number of eyes or number of fingers is not a variable. See quantitative variable, qualitative variable, discrete variable, continuous variable, dependent variable, and independent variable.
Variance	A measure of dispersion or scatteredness of quantitative data obtained as average of the squared deviations from mean.
Virulence	The ability to produce severe form of disease that can threaten life. Rabies is a highly virulent disease and cholera is not
Volunteer studies	A study done on volunteers, opposed to randomly selected subjects. Results from such studies can be used to identify tolerated dose and some side-effects of a test regimen but can not be used to estimate efficacy.
Waist-height ratio	The ratio of waist circumference to height. This ratio realizes that people with more height can have more waist circumference without being obese.
Waist-hip ratio	The ratio of waist circumference to hip measurement: sometimes considered a more valid measure of obesity than body mass index. Waist-hip ratio measures only central obesity, whereas body mass index is for overall obesity including central obesity.
Washout period	In a cross-over trial, the period elapsed between withdrawal of first treatment, and start of the second treatment. This period allows time for any effect of the first treatment to vanish before the second is started.
Wilcoxon test	Another method to do Mann-Whitney test for comparing central tendency of two groups. Both give exactly same result, and they are algebraically equivalent.
Zelen design	Under this design, the patients in a clinical trial randomized to a group are asked for consent to be in that group. If consent is not given, the patient goes to another group. This is considered more ethical but no blinding can done and the analysis of data is difficult.
z-score	Same as normal deviate but sometimes measured from median instead of mean, particularly in assessing growth: difference of a value from group mean in terms of how many times of SD.
z-test	A statistical test of hypothesis based on Gaussian distribution, generally used to compare two means or two proportions.

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