Objective: It is widely believed that psychological treatment has little effect on more severely depressed patients. This study assessed whether pretreatment severity moderates psychological treatment outcome relative to controls by means of meta-analyses.
Method: We included 132 studies (10,134 participants) from a database of studies (www.evidencebasedpsychotherapies.org) in which the effects of psychological treatment on adult outpatients with a depressive disorder or an elevated level of depressive symptoms were compared with a control condition in a randomized controlled trial. Two raters independently extracted outcome data and rated study characteristics. We conducted metaregression analyses assessing whether mean pretreatment depression scores predicted psychological treatment versus control condition posttreatment effect size and subgroup analyses summarizing the results of studies reporting within-study analyses of depression severity and psychological treatment outcome.
Results: Psychological treatment was found to be consistently superior to control conditions (d = 0.40-0.88). We found no indication that pretreatment mean depression scores predicted psychological treatment versus control condition posttreatment effect size, even after adjusting for relevant study characteristics. However, among the smaller subset of studies that reported within-study severity analyses, posttreatment effect sizes were higher for high-severity patients (d = 0.63) than for low-severity patients (d = 0.22) when psychological treatment was efficacious relative to a more stringent control.
Conclusion: Contrary to conventional wisdom, our findings suggest that when compared with control conditions, psychological treatment might be more efficacious for high-severity than for low-severity patients. Because the number of studies reporting within-study severity analyses is small, we recommend that future studies routinely report tests for Severity × Treatment interactions.
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