Background: No meta-analytical study has examined whether the quality of the studies examining psychotherapy for adult depression is associated with the effect sizes found. This study assesses this association.
Method: We used a database of 115 randomized controlled trials in which 178 psychotherapies for adult depression were compared to a control condition. Eight quality criteria were assessed by two independent coders: participants met diagnostic criteria for a depressive disorder, a treatment manual was used, the therapists were trained, treatment integrity was checked, intention-to-treat analyses were used, N >or= 50, randomization was conducted by an independent party, and assessors of outcome were blinded.
Results: Only 11 studies (16 comparisons) met the eight quality criteria. The standardized mean effect size found for the high-quality studies (d=0.22) was significantly smaller than in the other studies (d=0.74, p<0.001), even after restricting the sample to the subset of other studies that used the kind of care-as-usual or non-specific controls that tended to be used in the high-quality studies. Heterogeneity was zero in the group of high-quality studies. The numbers needed to be treated in the high-quality studies was 8, while it was 2 in the lower-quality studies.
Conclusions: We found strong evidence that the effects of psychotherapy for adult depression have been overestimated in meta-analytical studies. Although the effects of psychotherapy are significant, they are much smaller than was assumed until now, even after controlling for the type of control condition used.