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Home > Standards & Guidances > Methodological Guide

ENCePP Guide on Methodological Standards in Pharmacoepidemiology


5.1. Overview


An epidemiological study measures a parameter of occurrence (generally incidence, prevalence or risk or rate ratio) of a health phenomenon (e.g. a disease) in a specified population and with a specified time reference (time point or time period). Epidemiological studies may be descriptive or analytic. Descriptive studies do not aim to evaluate a causal relationship between a population characteristic and the occurrence parameter and generally do not include formal comparisons between population groups. Analytic studies, in contrast, use study populations assembled by the investigators to assess relationships that may be interpreted in causal terms. In pharmacoepidemiology, analytic studies generally aim to quantify the association between a drug exposure and a health phenomenon and test the hypothesis of a causal relationship. They are comparative by nature, e.g. comparing the occurrence of an outcome between subjects being drug users or being non-users or users of a different medicinal product.


Studies can be experimental or non-experimental (observational). Observational Studies: Cohort and Case-Control Studies (Plast Reconstr Surg. 2010;126(6):2234-42) provides a simple and clear explanation of the different types of studies and of their advantages and disadvantages. In experimental studies, the subjects are randomly assigned by the investigator to be either exposed or unexposed. These studies, known as randomised clinical trials (RCTs), are typically done to test the efficacy of treatments such as new medications. In RCTs, randomisation is used with the intention that the only difference between the exposed and unexposed groups will be the treatment itself. Thus, any differences in the outcome can be attributed to the effect of such treatment. In contrast to experimental studies where exposure is assigned by the investigator, in observational studies the investigator plays no role with regards to which subjects are exposed and which are unexposed. The exposures are either chosen by, or are characteristics of, the subjects themselves.


In order to obtain valid estimates of the effect of a determinant on a parameter of disease occurrence, analytic studies must address three types of epidemiological errors: random error (chance), systematic error (bias) and confounding.

  • Random error (chance): the observed effect estimate is a numerical value obtained from the study data which may be explained by random error because of the underlying variation in the population. The confidence interval (CI) allows the investigator to estimate the range of values within which the actual effect is likely to fall. 
  • Systematic error (bias): the observed effect estimate may be due to systematic error in the measurement of the exposure or disease, or in the selection of the study population. Systematic errors are often predictable. For example, mothers of children with congenital malformations will recall more instances of drug use during pregnancy than mothers of healthy children. This is known in epidemiology as “recall bias”, a type of information bias. Two main types of biases are generally described, selection bias and information bias. Information biases can occur whenever there are errors in the measurement of subject characteristics, for example a lack of pathology results leading to outcome misclassification of certain types of tumours, or lack of validation of exposure, leading to misclassification. The consequences of these errors depend on whether the distribution of errors for the exposure or disease depends on the value of other variables (differential misclassification) or not (nondifferential misclassification). Selection biases result from procedures used to select subjects and from factors that influence study participation, for example a researcher selecting controls in a way that they are associated with one dimension of the exposure of interest and do not represent the source population, or external factors such as media attention to safety issues that would influence health seeking behaviors and measurement of the incidence of a given outcome. 
  • Confounding: Confounding results from the presence of an additional factor, known as a confounder or confounding factor, that is associated with both the exposure of interest and the outcome. As a result, the exposed and unexposed groups will likely differ not only with regards to the exposure of interest, but also with regards to a number of other characteristics, some of which are themselves related to the likelihood of developing the disease. Confounding distorts the observed effect estimate for the outcome and the exposure under study. As there is not always a firm distinction between bias and confounding, confounding is also often classified as a type of bias.

There are many different situations where bias may occur, and some authors give a name to each of them. The number of such situations is in theory illimited and, rather than being able to name each of them, it is preferable to understand the underlying mechanisms of information bias, selection bias and confounding, be alert to their presence and likelihood of occurrence in a study and recognise methods for their prevention, detection and control at the analytical stage if possible, such as restriction, stratification, matching, regression and sensitivity analyses. Chapter 6.1 on methods to address bias and confounding nevertheless treats time-related bias (a type of information bias with misclassification of person-time) separately as they may have important consequences on the result of a study and may be dealt with by design and time-dependent analyses.


The large number of observational studies performed urgently with existing data and in sometimes difficult conditions during the COVID-19 pandemic has raised concerns about the validity of many studies published without peer-review. Considerations for pharmacoepidemiological analyses in the SARS-CoV-2 pandemic (Pharmacoepidemiol Drug Saf. 2020;29(8):825-83) provides recommendations across eight domains: (1) timeliness of evidence generation; (2) the need to align observational and interventional research on efficacy (3) the specific challenges related to “real‐time epidemiology” during an ongoing pandemic; (4) which design to use to answer a specific question; (5) considerations on the definition of exposures and outcomes and what covariates to collect ; (6) the need for transparent reporting; (7) temporal and geographical aspects to be considered when ascertaining outcomes in COVID-19 patients, and (8) the need for rapid assessment. The article Biases in evaluating the safety and effectiveness of drugs for covid-19: designing real-world evidence studies.(Am J Epidemiol. 2021;kwab028) reviews and illustrates how immortal time bias and selection bias were present in several studies evaluating the effects of drugs on SARS-CoV-2 infection, and how they can be addressed.



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