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Clinical Epidemiology Glossary



Absolute risk: The observed or calculated probability of an event in the population under study.
(Harm/Etiology, Therapy)

Absolute risk difference: The difference in the risk for disease or death between an exposed population and an unexposed population. (Harm/Etiology)

Absolute risk reduction (ARR): the difference in the absolute risk (rates of adverse events) between study and control populations. (Therapy) To Calculation

Adjustment: A summarizing procedure in which the effects of differences in composition of the populations being compared have been minimized by statistical methods. See Confounding variable(Harm/Etiology)

Association: Statistical dependence between two or more events, characteristics, or other variables. An association may be fortuitous or may be produced by various other circumstances; the presence of an association does not necessarily imply a causal relationship. (Harm/Etiology)


Bias (systematic error): Deviation of results or inferences from the truth, or processes leading to such deviation.

See also Referral bias, Selection bias, Verification bias.
(Harm/Etiology, Therapy)

Blind assessment: The evaluation of an outcome is made without the evaluator  knowing which results are from the test under study and which are from the control or “gold standard”.

Blind(ed) study (masked study): A study in which observer(s) and/or subjects are kept ignorant of the group to which the subjects are assigned, as in an experimental study, or of the population from which the subjects come, as in a nonexperimental or observational study. Where both observer and subjects are kept ignorant, the study is termed a double-blind study. If the statistical analysis is also done in ignorance of the group to which subjects belong, the study is sometimes described as triple blind. The purpose of "blinding" is to eliminate sources of bias. (Diagnosis, Harm/Etiology, Therapy)


Case-series: Report of a number of cases of disease. (Harm/Etiology)

Case-control study: Retrospective comparison of exposures of persons with disease (cases) with those of persons without the disease (controls) (Harm/Etiology). See Retrospective study.

Causality: The relating of causes to the effects they produce. Most of epidemiology concerns causality and several types of causes can be distinguished. It must be emphasized, however, that epidemiological evidence by itself is insufficient to establish causality, although it can provide powerful circumstantial evidence. (Harm/Etiology)

Clinical outcome: Measures patient health or well being. Ideally it should be credible, comprehensive, sensitive to change, accurate, sensible, and biologically sensible. Compare with Surrogate outcome. (Diagnosis, Harm/Etiology, Prognosis, Therapy)

Co-interventions: Interventions other than the treatment under study that may have been applied differently to the study and control groups. Co-intervention is a serious problem when double-blinding is absent or when the use of very effective non-study treatments is permitted. (Therapy)

Cohort study: A study that begins with the gathering of two matched groups (the cohorts), one which has been exposed to a prognostic factor,risk factor or intervention and one which has not. The groups are then followed forward in time (prospective) to measure the development of different outcomes. In a retrospective cohort study, cohorts are identified at a point of time in the past and information is collected on their subsequent outcomes.
(Diagnosis, Harm/Etiology, Prognosis, Therapy)

An inception cohort is a group identified at the onset of a disorder or a first exposure to a potential risk factor, and followed forward in time. (Harm/Etiology, Prognosis)

Comparison group: Any group to which the index group is compared. Usually synonymous with control group. (Harm/Etiology, Therapy)

Co-morbidity: Coexistence of a disease or diseases in a study participant in addition to the index condition that is the subject of study. (Harm/Etiology, Prognosis, Therapy)

Confidence interval (CI): "The CI gives a measure of the precision (or uncertainty) of study results for making inferences about the population of all such patients". (Strauss, 2005 p. 263) The 95% CI is the range of values within which we can be 95% sure that the true value lies for the whole population of patients from whom the study patients were selected. Wide confidence intervals indicate less precise estimates of effect. CI is affected by sample size and by variability among subjects. The larger the trial's sample size is, the larger the number of outcome events and the greater the confidence that the true relative risk reduction is close to the value stated: the confidence intervals narrow and "precision" is increased.
(Harm/Etiology, Therapy) To Calculation

Confounding variable (confounder): A characteristic that may be distributed differently between the study and control groups and that can effect the outcome being assessed.  Confounding may be due to chance or bias.  See Adjustment, Selection bias
(Harm/Etiology, Therapy)

Control event rate (CER): The percentage of the control/nonexposed group who experience outcome. (Harm/Etiology, Therapy)
To Calculation


Dose-response relationship: A relationship in which change in amount, intensity, or duration of exposure is associated with a change--either an increase or decrease--in frequency or intensity of a specified outcome. (Harm/Etiology)

Determinant: Any definable factor that effects a change in a health condition or other characteristic. (Harm/Etiology)


Effectiveness: A measure of the benefit resulting from an intervention administered under usual conditions of clinical care for a particular group of patients. See Intention to treat. (Therapy)

Efficacy: A measure of the benefit resulting from an intervention for a given health problem administered to patients under ideal conditions (i.e., perfect compliance). (Therapy)

Etiology: The study of the cause or origin of a disease.

Exclusion criteria: Stated conditions which preclude entrance of candidates into an investigation even if they meet the inclusion criteria. (Diagnosis, Harm/Etiology, Prognosis, Therapy)

Experimental event rate (EER): The percentage of intervention/exposed group who experience outcome in question.
To Calculation


Follow-up: Observation over a period of time of an individual, group, or initially defined population whose relevant characteristics have been assessed in order to observe changes in health status or health-related variables. (Harm/Etiology, Prognosis, Therapy)


Gold standard (also Reference standard): Ideally, the criterion used to unequivocally define the presence of a condition; or practically, the method, procedure or measurement that is widely accepted as being the best available to detect the presence of a condition. (Diagnosis)


Incidence: The rate of new cases of illness commencing during a specified time period in a given population See also Prevalence. (Harm/Etiology)

Index test: The test whose diagnostic accuracy is being measured against the reference or gold standard. (Diagnosis)

Intention to treat analysis: Individual outcomes in a clinical trial are analyzed according to the group to which they have been randomized, regardless of whether they dropped out, fully complied with the intervention or crossed over to the other treatment. By simulating practical experience intention to treat analysis provides a better measure of effectiveness (as opposed to efficacy). (Therapy)

Interviewer bias: Systematic error due to interviewer's subconscious or conscious gathering of selective data. (Harm/Etiology)


Lead-time bias: : Overestimation of survival because of earlier diagnosis—time of death does not change, just time of diagnosis. (Harm/Etiology, Prognosis)

Likelihood ratio: The likelihood ratio for a test result compares the likelihood of that result in patients with disease to the likelihood of that result in patients without disease.
arrowLikelihood ratio of a negative test: Ratio of the probability of a false negative result if the disease is present to the probability of a true negative result if the disease is absent.

arrowLikelihood ratio of a positive test: Ratio of the probability of a true positive result if the disease is present to a false positive result if the disease is absent.(Diagnosis)
To Calculation


Meta-analysis: Statistical synthesis of the results from several studies that address the same question.


Negative predictive valueThe proportion of people who receive a negative test result who are truly free of the target disorder.

Number needed to treat (NNT): The number of patients with a particular condition who must receive an intervention to prevent the occurrence of one adverse outcome.
(Therapy)To Calculation

arrowNumber needed to harm (NNH): The number of patients for whom there is one additional patient who experiences a harmful outcome. Calculated the same way as NNT.


Observational study (non-experiemental study): Changes or differences in one characteristic (e.g. whether or not people received a specific treatment or intervention) are studied in relation to changes or differences in other(s) (e.g. whether or not they died), without the intervention of the investigator. Examples are case control and cohort studies.

Odds: A ratio between two probabilities—the probability of an event to a non-event. (Etiology/Harm, Therapy)

Odds ratio (OR): The odds of the experimental group showing positive (or negative) effects of an intervention or exposure, in comparison to the control group. (Etiology/Harm, Therapy)
To Calculation


P value:  The possibility that any particular outcome would have occurred by chance. Statistical significance is usually p<0.05. Considered to be inferior to confidence intervals in determining significance of studies.

Power:The ability of a study to demonstrate an association or causal relationship between two variables, given that an association exists. For example, 80% power in a clinical trial means that the study has a 80% chance of showing a statistically significant treatment effect if there really was an important difference between outcomes. If the statistical power of a study is low, the study results will be questionable (the study might have been too small to detect any differences).
By convention, 80% is an acceptable level of power. (Bandolier; April 1, 2008. http://www.jr2.ox.ac.uk/bandolier/booth/glossary/statpow.html )

Precision (statistical precision): The range in which the best estimates of a true value approximate the true value.
(Diagnosis, Harm/Etiology, Prognosis, Therapy)
See Confidence interval.

Predictive value: In screening and diagnostic tests, the probability that a person with a positive test is a true positive (i.e., does have the target disease), or that a person with a negative test truly does not have the disease. The predictive value of a screening test is determined by the sensitivity and specificity of the test, and by the prevalence of the condition for which the test is used.
(Diagnosis) To Calculation

Prevalence: The proportion of persons with a particular disease within a given population at a given time. (Diagnosis)

Primary research:  Individual studies such as randomized controlled trials, cohort studies, case-control studies, cross-sectional studies, etc.

Prognosis: the possible outcomes of a disease or condition and the likelihood that each one will occur. (Prognosis)

Prognostic factor: A factor or indicator (such as age or gender) related to an individual’s probability of developing a disease or other outcome.  Compare with risk factors. Neither prognostic nor risk factorsnecessarily imply a cause and effect relationship. (Prognosis)

Prospective study: Study design where one or more groups (cohorts) of individuals who have not yet experienced the outcome event in question are followed forward in time and monitored for the number of such events which occur (Diagnosis, Harm/Etiology, Prognosis, Therapy)


Randomized controlled trial: An experimental comparison study in which participants are allocated via a randomization mechanism to either an intervention/treatment group or a control /placebo group, then followed over time and assessed for the outcomes of interest.  Participants have an equal chance of being allocated to either group. (Therapy)

Recall bias: Systematic error due to the differences in accuracy or completeness of recall to memory of past events or experiences. (Harm/Etiology)

Reference standard: See Gold standard.

Referral bias: The sequence of referrals that may lead patients from primary to tertiary centers raises the proportion of more severe or unusual cases, thus increasing the likelihood of adverse or unfavorable outcomes. Physicians and medical centers may attract individuals with specific disorders or exposures. (Prognosis)

Relative risk (RR): The ratio of the probability of developing, in a specified period of time, an outcome among those receiving the treatment of interest or exposed to a risk factor, compared with the probability of developing the outcome if the risk factor or intervention is not present. (Therapy, Harm/Etiology)
To Calculation

Relative risk reduction (RRR): The extent to which a treatment reduces a risk, in comparison with patients not receiving the treatment of interest. (Therapy) To Calculation

Reproducibility (repeatability, reliability): The results of a test or measure are identical or closely similar each time it is conducted. (Diagnosis)

Retrospective study: Study design in which cases where individuals who had an outcome event in question are collected and analyzed after the outcomes have occurred. (Harm/Etiology)
See also Case-control study.

Risk factor: Patient characteristics or factors associated with an increased probability of developing a condition or disease in the first place. Compare with prognostic factors. Neither risk nor prognostic factors necessarily imply a cause and effect relationship. (Harm/Etiology)


Sample size: Is the size of the sample. Larger samples usually mean more precise results. Sample size usually depends on the purpose of the study, the population size from which the sample the sample will be pulled, as well as the level of precision and the level of confidence or risk that is acceptable, and the degree of variability in the attributes being measured.

Selection bias: A bias in assignment or a confounding variable that arises from study design rather than by chance. These can occur when the study and control groups are chosen so that they differ from each other by one or more factors that may affect the outcome of the study. (Harm/Etiology, Therapy)

Sensitivity (of a diagnostic test): The proportion of truly diseased persons, as measured by the gold standard, who are identified as diseased by the test under study. (Diagnosis)
To Calculation

Sensitivity Analysis (economic studies): A technique for testing the robustness of a decision analysis by repeating the analysis with a range of probability and utility estimates.

Specificity (of a diagnostic test): The proportion of truly non-diseased persons, as measured by the gold standard, who are so identified by the diagnostic test under study. (Diagnosis)
To Calculation

Statistical significance: How likely the result is due to chance. The probability that an event or difference occurred by chance alone.

Stratification: Division into groups. Stratification may also refer to a process to control for differences in confounding variables, by making separate estimates for groups of individuals who have the same values for the confounding variable. (Therapy)

Strength of inference: The likelihood that an observed difference between groups within a study represents a real difference rather than mere chance or the influence of confounding factors, based on both p values and confidence intervals. Strength of inference is weakened by various forms of bias and by small sample sizes. (Harm/Etiology, Therapy)

Surrogate outcome/endpoint: Intended to capture the treatment effect of an important clinical endpoint but does not directly measure the clinical benefit of the intervention, substitutes something we can measure for something we want to measure. Compare with clinical outcome. (Diagnosis, Harm/Etiology, Prognosis, Therapy)

Survival curve: A graph of the number of events occurring over time or the chance of being free of these events over time. The events must be discrete and the time at which they occur must be precisely known. In most clinical situations, the chance of an outcome changes with time. In most survival curves the earlier follow-up periods usually include results from more patients than the later periods and are therefore more precise. (Prognosis)


Test/treatment thresholds: The probability of disease above which we treat for the disease and below which we do not treat. The treatment threshold is determined by the costs and benefits of the treatment.

Values can be assigned to these thresholds from data on the reliability and potential risks of the diagnostic test and the benefits and risks of the diagnostic test and the benefits and risks of a specific treatment. Treatment should be withheld if the probability of disease is smaller than the testing threshold, and treatment should be given without further testing if the probability of the disease is greater than the test-treatment threshold. The test should be performed (with treatment depending on the test outcome) only if the probability of disease is between the two thresholds. The method exposes important principles of decision making and helps the clinician develop a rational, quantitative approach to the use of diagnostic tests.

The probability at which one should be indifferent between testing and treating.


Validity: The degree to which the results of a study are likely to be true, believable and free of bias. This is entirely independent of the precision of the results and does not predict the results to your patients. (Diagnosis, Harm, Prognosis, Therapy)

arrowThe internal validity of a study refers to the integrity of the experimental design.

arrowThe external validity of a study refers to the appropriateness by which its results can be applied to non-study patients or populations.

Verification bias (work-up bias): Occurs when patients with negative test results are not evaluated with the gold standard test. See also Bias.