The use and abuse of subgroup analysis in epidemiological research.

Most epidemiology involves the analysis of subgroups. In observational studies, identification of associations within particular subgroups is the usual method of investigation, while in experimental studies the purpose is to compare the rate of occurrence of the outcome of interest in treated and control groups. Problems can arise through failure to specify the subgroups of interest a priori and through examining large numbers of subgroups after the fact, i.e., through multiple testing. Rules for interpretation of findings in subgroups are suggested, and the value of the systematic application of criteria for judgement of the causal significance of any associations that may be observed is noted.