Comparison With Other Studies
The pooled estimate of all 29 studies providing posttreatment depression score data in our review (SMD = −0.29) is similar to the pooled estimate found in a meta-analysis from 2009 that included 15 trials (SMD = −0.31).15 This effect of treatment compared with usual care or placebo must be considered as relatively small according to usual standards.22,26 Nevertheless, usual care should not be equated to no treatment or a wait list control. A recent network meta-analysis by Barth et al of 198 trials not limited to primary care patients showed that compared with a wait list control, both usual care and placebo interventions were associated, with a SMD of −0.33, and the SMD of psychological treatments compared with wait list controls was −0.62, which would be considered to be a moderate or large effect.8 The amount and robustness of evidence for individual treatment approaches varied, which is similar to our findings, but there was little evidence that treatment effects differed. Barth et al also found that larger trials yielded less optimistic findings than smaller trials. Another meta-analytic study of a similar data set concluded that the effects of psychological treatments for adult depression are likely overestimated because of publication bias.27 The funnel plot in our review looks asymmetrical, but pooled estimates in larger and smaller studies included in our review differed only marginally. Even so, we cannot rule out that the effects of psychological treatments in our meta-analysis are to some extent overestimated.
We could not confirm the finding of the meta-analysis by Cuijpers et al that effects of psychological treatments are smaller in trials in which participants are recruited by screening,15 but we did find that the effects were smaller in trials in patients with minor depression or dysthymia. Other studies point to a similar direction.28,29 Still, mixing acute and persistent forms of minor depression may have masked important clinical variation, as responsiveness to psychotherapeutic treatments is likely to vary according to the persistence of symptoms.18 When interpreting our findings, it must be kept in mind that 5 of the 6 trials focusing on dysthymia or minor depression in our meta-analysis investigated problem-solving therapy. In our subgroup analyses we also found a trend that psychological treatments were slightly less effective in the elderly, but this finding might be by chance, and an earlier, large meta-analysis by Cuijpers et al not restricted to primary care did not find a significant difference in effects between younger and older adults.30
The intensity of treatment in usual care groups and the level of usual care interventions in groups receiving psychological treatments varied, which can have a major impact on the effect estimates. Only one-third of the trials were considered to have a low risk of bias, and publication bias seems at least possible. The reporting of treatment discontinuation, dropout from the study, and loss to follow-up rarely provided sufficient detail to assess whether attrition was due to acceptability of the treatment or organizational problems. No study reported adverse events or adverse effects for psychological treatments (not even studies including a pharmacotherapy treatment group). As a result, we could not assess acceptability and feasibility of treatments to the extent planned in our protocol.17 In summary, given the limited number, rather low quality, and considerable heterogeneity of the available studies, the findings of our review have to be interpreted carefully.