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Introduction
In the past three decades more than 100 randomized controlled studies have shown that psychotherapy is an effective treatment of depressive disorders in adults. Meta-analyses in this field have consistently found that psychotherapies have moderate to large effect sizes (Gloaguen et al. 1998; Churchill et al. 2001; Wampold et al. 2002; Cuijpers et al. 2007a; Ekers et al. 2008). However, in several adjacent fields recent meta-analytical studies have found strong indications that earlier meta-analytical studies have overestimated treatment effects considerably. In two recent meta-analytical reviews of studies examining the effects of antidepressive medication, it was found that these effects have been overestimated because of publication bias (Kirsch et al. 2008; Turner et al. 2008). Recent meta-analyses in the field of psychotherapy for depression in children and adolescents also found indications that the effects have been overestimated considerably in earlier meta-analytical research (Weisz et al. 2006; Klein et al. 2007).
One of the major reasons for overestimation of the overall efficacy of an intervention is the inclusion in meta-analyses of studies with a low quality (Altman, 2002). Whether the quality of studies has a negative impact on the mean effect sizes found in meta-analyses of psychotherapy for adult depression has rarely been addressed. Most meta-analyses in this field have focused on specific types of psychotherapy (De Mello et al. 2005; Cuijpers et al. 2007b; Leichsenring & Rabung, 2008), specific target groups (Bledsoe & Grote, 2006; Pinquart et al. 2006), or specific delivery formats (McDermut et al. 2001; Gellatly et al. 2007), and only one of these studies even addressed the association between effect size and study quality (Gellatly et al. 2007). In that study, significant associations between the effect size and some quality criteria, including concealment of allocation and intention-to-treat analyses, were found with respect to self-help treatments for depression. In another meta-analysis on psychodynamic therapies a correlation was calculated between effect sizes and study quality as a continuous measure, but no significant association was found (Leichsenring & Rabung, 2008). Only a few meta-analyses have focused on the complete field of psychotherapy for adult depression, and these have not examined study quality at all (Robinson et al. 1990), or have not examined an association...