Rudiments of subgroup analyses.

Subgroup analysis in a clinical trial is the evaluation of the effect of a randomly allocated intervention within only a fraction of the patients in the entire research cohort. This article provides several examples of the use of subgroup analysis, discusses some of the interpretative difficulties that occur during the assessment of the effect of therapy within subgroups, and provides a summary of recent recommendations on reporting subgroup analyses in the literature. Although subgroup analyses can provide new, provocative, and sometimes clinically relevant findings, this group of evaluations must be handled with extreme care.