Assessing treatment effect moderation in trials of psychological interventions: a case for individual participant data meta-analysis of pooled trials

The investigation of treatment effect moderation is a common research objective in psychological studies. The benefit or harm of an intervention might vary within a study according to patient characteristics or between studies due to contextual factors such as the composition of the study population or the study setting. Statistically the investigation of treatment effect moderation amounts to an assessment of interactions between putative moderators and treatment groups. Interaction tests tend to be underpowered when based on individual participant data (IPD) from a single study. Pooling multiple trials aimed at evaluating the same intervention offers a way forward her