Are all psychotherapies equally effective in the treatment of adult depression? The lack of statistical power of comparative outcome studies.

More than 100 comparative outcome trials, directly comparing 2 or more psychotherapies for adult depression, have been published. We first examined whether these comparative trials had sufficient statistical power to detect clinically relevant differences between therapies of d=0.24. In order to detect such an effect size, power calculations showed that a trial would need to include 548 patients. We selected 3 recent meta-analyses of psychotherapies for adult depression (cognitive behaviour therapy (CBT), interpersonal psychotherapy and non-directive counselling) and examined the number of patients included in the trials directly comparing other psychotherapies. The largest trial comparing CBT with another therapy included 178 patients, and had enough power to detect a differential effect size of only d=0.42. None of the trials in the 3 meta-analyses had enough power to detect effect sizes smaller than d=0.34, but some came close to the threshold for detecting a clinically relevant effect size of d=0.24. Meta-analyses may be able to solve the problem of the low power of individual trials. However, many of these studies have considerable risk of bias, and if we only focused on trials with low risk of bias, there would no longer be enough studies to detect clinically relevant effects. We conclude that individual trials are heavily underpowered and do not even come close to having sufficient power for detecting clinically relevant effect sizes. Despite this large number of trials, it is still not clear whether there are clinically relevant differences between these therapies. Published by the BMJ Publishing Group Limited. For permission to use (where not already granted under a licence) please go to http://www.bmj.com/company/products-services/rights-and-licensing/