Case-control studies.

This paper will provide an overview of the case-control study including the definition, types of study design, advantages and disadvantages, potential types and sources of bias, selection of controls, statistical measures of association, and criteria for suggesting inferences about causation. Measures of association for HLA-B27 and ankylosing spondylitis (AS) and HLA-DR4 and rheumatoid arthritis (RA) are calculated from previously published data. Case-control studies are useful for identifying etiologic associations in uncommon diseases.