Causal meditation analysis to understand how different components of a complex psychological intervention improved symptoms (or not) of depression in Goa, India

BackgroundUnderstanding how and under what circumstances a highly effective psychological intervention, improved symptoms of depression is important to bring this evidence-informed intervention to scale, particularly in resource-poor settings. We aim to estimate the indirect effects of potentially important mediators to improve symptoms of depression in the Healthy Activity Program (HAP) trial.MethodsInterventional in(direct) effects were used to simultaneously decompose the total effect of the intervention on depression symptoms measured through the Patient Health Questionnaire (PHQ-9). The following indirect effects were considered: characteristics of sessions including the number of sessions and homework completed; behavioural activation according to an adapted version of the Behavioural Activation for Depression Short Form (BADS-SF), and extra sessions offered to participants who did not respond to the intervention.ResultsOf the total effect of the intervention measured through the difference in PHQ-9 scores between treatment arms (mean difference: -2.2, 95% bias-corrected CI: -3.2, -0.8), 45% was mediated through improved levels of behavioural activation (−1.0, -1.3, -0.6). There was no evidence to support the mediating role of characteristics of the sessions nor the extra sessions offered to participants who did not respond to the treatment.ConclusionsFindings from our robust mediation analyses, confirmed the importance of behavioural activation in improving depression symptoms. Contrary to published literature, our findings suggest that neither the number of sessions nor proportion of homework completed, improved outcomes. Moreover, results indicate that the extra sessions were insufficient to improve symptoms of depression for participants who did not respond to the intervention.