• asymptotic relative efficiency;
  • clinical trials;
  • combined outcomes;
  • composite endpoints;
  • log-rank test

When comparing two treatment groups in a time-to-event analysis, it is common to use a composite event consisting of two or more distinct outcomes. The goal of this paper is to develop a statistical methodology to derive efficiency guidelines for deciding whether to expand a study primary endpoint from inline image (for example, non-fatal myocardial infarction and cardiovascular death) to the composite of inline image and inline image (for example, non-fatal myocardial infarction, cardiovascular death or revascularisation). We investigate this problem by considering the asymptotic relative efficiency of a log-rank test for comparing treatment groups with respect to a primary relevant endpoint inline image versus the composite primary endpoint, say inline image, of inline image and inline image, where inline image is some additional endpoint. Copyright © 2012 John Wiley & Sons, Ltd.