title: Case-control studies in clinical research: mechanism and prevention of selection bias.
abstract: The mechanism by which selection bias occurs in case-control studies is explained to an audience of clinicians using a simple conceptual framework and a graphical presentation. A case-control study consists in comparing the frequency of exposure in a group of subjects having the studied disease (the cases) relative to another group free of that disease (the controls). Cases and controls can be thought of as arising from a hypothetical cohort study. Thus, enrolled cases are a fraction F1 of the exposed who developed the disease plus a fraction F3 of the unexposed who developed the disease during a given period. Similarly, enrolled controls are a fraction F2 of the exposed who did not develop disease plus a fraction F4 of the unexposed who did not develop the disease. A selection process is inherent to the design of case-control studies but it leads to selection bias only when the ratio of F1 x F4/F2 x F3 is not equal to unity. Examples demonstrate the implication of sampling fractions for designing and interpreting case-control studies performed in clinical settings.
authors.affiliation: Division of Clinical Epidemiology, University Hospitals of Geneva, Switzerland.
authors.forename: A
authors.initials: A
authors.lastname: Morabia
mesh_terms: D016022:Case-Control Studies; D015331:Cohort Studies; D006801:Humans; D016017:Odds Ratio; D018579:Patient Selection; D012017:Referral and Consultation; D012107:Research Design / Q000592:standards*; D018401:Sample Size; D012494:Sampling Studies; D015983:Selection Bias