The choice of the most appropriate primary and secondary outcome measures is often the most complex issue in the design of a randomised controlled clinical trial. It has implications for the cost of the trial, the sample size, the burden that the trial will place on patients and clinicians taking part, and the likelihood that the result of the trial will influence clinical practice. As illustrated by Freeman et al 1 in the previous issue of this Journal (February pp 150–6), whichever outcome is chosen it is important that it has been properly validated in a representative sample of patients with the disease under study. They assessed whether the short form 36 (SF-36), the most often used generic measure of health status, has the properties necessary to detect clinically significant …