Recent years have seen major developments in psychotherapy research that suggest the need to address critical methodological issues. These recommendations, developed by an international group of researchers, do not replace those for randomized controlled trials, but rather supplement strategies that need to be taken into account when considering psychological treatments. The limitations of traditional taxonomy and assessment methods are outlined, with suggestions for consideration of staging methods. Active psychotherapy control groups are recommended, and adaptive and dismantling study designs offer important opportunities. The treatments that are used, and particularly their specific ingredients, need to be described in detail for both the experimental and the control groups. Assessment should be performed blind before and after treatment and at long-term follow-up. A combination of observer- and self-rated measures is recommended. Side effects of psychotherapy should be evaluated using appropriate methods. Finally, the number of participants who deteriorate after treatment should be noted according to the methods that were used to define response or remission.

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BackgroundPsychotherapy is an evidence-based treatment for depression, but its average effect is modest. Thus, identifying subgroups that respond especially well to psychotherapy is an important goal. This would allow maximizing the efficacy of interventions. However, the extent of treatment effect heterogeneity (TEH) has not yet been systematically investigated. A reliable, evidence-based estimate of this heterogeneity would allow a more accurate assessment of the potential effects of enhancement by personalization.MethodsUsing a large database of randomized-controlled trials on psychotherapy for depression in adults (k = 306), we performed a Bayesian variance ratio meta-regression. Based on the results, we determined the theoretically maximum possible extent of variability of individual outcomes. Exploratory subgroup analyses were conducted for different types of psychotherapy. We determined the extent of expected TEH given the evidence by employing an analytical approach.OutcomesWe found for the entire sample a 9% higher variance in the intervention groups compared to the control groups, indicating TEH in psychotherapy for depression. Depending on the depression scale used, this corresponds to a standard deviation of the individual treatment effect of 3-4 points. Subgroup analyses revealed that due to a large number of studies, the evidence for TEH is strongest in cognitive-behavioural therapy, while the largest TEH was observed in behavioural activation therapy.InterpretationOur results show that the treatment effect of psychotherapy for depression varies. This is a clear indication that patients benefit differently from different interventions. Clinical or statistical methods that select the optimal intervention for each patient are likely to increase the success of treatment.

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