Biostatistics primer: what a clinician ought to know: subgroup analyses.

Large randomized phase III prospective studies continue to redefine the standard of therapy in medical practice. Often when studies do not meet the primary endpoint, it is common to explore possible benefits in specific subgroups of patients. In addition, these analyses may also be done, even in the case of a positive trial to find subsets of patients where the therapy is especially effective or ineffective. These unplanned subgroup analyses are justified to maximize the information that can be obtained from a study and to generate new hypotheses. Unfortunately, however, they are too often overinterpreted or misused in the hope of resurrecting a failed study. It is important to distinguish these overinterpreted, misused, and unplanned subgroup analyses from those prespecified and well-designed subgroup analyses. This overview provides a practical guide to the interpretation of subgroup analyses.