a1 Department of Clinical Psychology, VU University Amsterdam, The Netherlands
a2 EMGO Institute, VU University Medical Center, Amsterdam, The Netherlands
a3 Technical University Twente, Deventer, The Netherlands
a4 Department of Psychology, Vanderbilt University, Nashville, TN, USA
a5 Department of Behavioural Sciences and Learning, Swedish Institute for Disability Research, Linköping University, Sweden
a6 Department of Clinical Neuroscience, Psychiatry Section, Karolinska Institutet, Stockholm, Sweden
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, N50, 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.
(Received December 02 2008)
(Revised March 16 2009)
(Accepted April 29 2009)
(Online publication June 03 2009)
c1 Address for correspondence: P. Cuijpers, Ph.D., Professor of Clinical Psychology, Department of Clinical Psychology, VU University Amsterdam, Van der Boechorststraat 1, 1081 BT Amsterdam, The Netherlands. (Email: email@example.com)