Case-control studies.

Case-control studies investigate associations between exposures to risk factors and a condition of interest. Subjects with the condition (cases) are compared to subjects known not to have the condition (controls) with respect to the presence of risk factors in the past. This design is particularly useful when the condition of interest is relatively uncommon. Case-control studies are useful for studying the etiology of conditions in primary care. Invalid results may be obtained, however, if a study's design allows the occurrence of selection bias, information bias, or confounding.