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Case-control studies, which are always retrospective, compare those who have had an outcome or event () with those who have not (). Cases and controls are then evaluated for exposure to various and thus should never be selected on the basis of their exposure to the risk factors under investigation. Cases and controls generally are matched according to specific characteristics (eg, age, sex, or duration of disease) to reduce by these variables. However, if the matched variables are inextricably linked with the exposure of interest (not necessarily with the disease or outcome of interest), matching may confound the analysis (see Less