Selection bias and treatment heterogeneity in clinical trials.

A common perception about many commercially available medical treatments is that they are effective for every patient having the relevant indication and that developers have provided the regulatory authorities with evidence of such a property. We show that the standard of evidence is much lower and that the standard is appropriate only when the treatment effects are almost constant. We discuss the implications on the design and analysis of clinical trials if the standards were made to correspond with the common perception. We conclude that the evidence of positive mean treatment effect should be accompanied by evidence of limited dispersion of the effects and by a sensitivity analysis that explores the impact of the selection bias in recruitment. Copyright 1999 John Wiley & Sons, Ltd.