Clinical trials represent a major investment by investigators, sponsors and participants, and it is reasonable to attempt to gain the maximum information from them. Practitioners and regulatory agencies are keen to know whether there are subgroups of trial participants who are more (or less) likely to be helped (or harmed) by the intervention under investigation, and a recent survey of trials published over 3 months in four leading journals found that 70% included subgroup analyses.1,2 Furthermore, regulatory guidance documents (such as the Committee for Proprietary Medicinal Products September 2002 document Points to consider on multiplicity issues in clinical trials3) strongly encourage appropriate subgroup analyses. The results of subgroup analyses can also drive changes in practice guidelines. For example, the United States National Institutes of Health issued a clinical alert following the unexpected finding in the BARI (Bypass Angioplasty Revascularisation Investigation) trial that mortality after angioplasty in patients with diabetes was nearly double that after bypass-graft surgery (P = 0.003).4
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