Comparative clinical trials in psychotherapy: Have large effects been replicated?

The purpose of this review is to examine the replication attempts of psychotherapy clinical trials for depression and anxiety. We focus specifically on replications of trials that exhibit large differences between psychotherapies. The replicability of these trials is especially important for meta-analysis, where the inclusion of false-positive trials can lead to erroneous conclusions about treatment efficacy.

Standard replication criteria were developed to distinguish direct from conceptual replication methodologies. Next, an exhaustive literature search was conducted for published meta-analyses of psychotherapy comparisons. Trials that exhibited large effects (d > 0.8) were culled from these meta-analyses. For each trial, a cited replication was conducted to determine if the trial had been subsequently replicated by either 'direct' or 'conceptual' methods. Finally, a broader search was conducted to examine the extent of replication efforts in the psychotherapy literature overall.

In the meta-analytic search, a total of N = 10 meta-analyses met the inclusion criteria. From these meta-analyses, N = 12 distinct trials exhibited large effect sizes. The meta-analyses containing more than two large effect trials reported evidence for treatment superiority. A cited replication search yielded no direct replication attempts (N = 0) for the trials with large effects, and N = 4 conceptual replication attempts of average or above average quality. However, of these four attempts, only two partially corroborated the results from their original trial.

Meta-analytic reviews are influenced by trials with large effects, and it is not uncommon for these reviews to contain several such trials. Since we find no evidence that trials with such large effects are directly replicable, treatment superiority conclusions from these reviews are highly questionable. To enhance the quality of clinical science, the development of authoritative replication criteria for clinical trials is needed. Moreover, quality benchmarks should be considered before trials are included in a meta-analysis, or replications are attempted.