Group analysis versus individual response: the inferential limits of randomized controlled trials

The randomized controlled trial (RCT) is the gold standard for assessing the efficacy of medical treatments. Over the past 50 years, RCT methodology has proven to be quite successful in identifying effective treatments and weeding out ineffective ones, thus transforming medicine from an intuitive art into an empirical science. However, the enormous success of the RCT has inadvertently contributed to a common inferential error that is insufficiently appreciated by some clinicians and researchers. Although RCTs can effectively distinguish between placebo and active treatment effects at the level of the group, contrary to intuition, this same disentanglement is much more difficult to achieve at the level of the individual. For individual patients it is surprisingly difficult to determine who is a treatment responder and who is not. Using data from a recent RCT, we illustrate the problem and detail its negative effects for research and clinical practice. Finally, we suggest strategies for minimizing these negative effects.