The case-control study. A practical review for the clinician.

The retrospective case-control study is an important research strategy commonly encountered in the medical literature. A thoughtfully designed, carefully executed case-control study can be an invaluable source of clinical information, and physicians must often base important decisions about patient counseling and management on their interpretation of such studies. Unfortunately, the retrospective direction of case-control studies--looking "backwards" from an outcome event to an antecedent exposure--is accompanied by numerous methodological hazards. Careful attention must be paid to selection of appropriate study groups; definition and detection of the outcome event; definition and ascertainment of the exposure; assurance that the compared groups were equally susceptible to the outcome event at baseline; and careful statistical analysis. If systematic bias enters the research at any of these points, erroneous conclusions can result. Greater familiarity with the case-control method should enable clinicians to be more critically insightful when interpreting the results of published studies using this design format.