A primer on investigating treatment effect heterogeneity and causal moderation in psychotherapy research

Experimental designs provide strong evidence for causal claims involving the main effects of a treatment but analyzes that solely focus on the main effects are inherently limited. Considerations for effect heterogeneity allow psychotherapy researchers to investigate for whom and under what conditions a treatment is likely to be effective. Evidence of causal moderation requires more stringent assumptions but is an important extension of treatment effect heterogeneity when interventions on the moderator are possible.

This primer clarifies and differentiates treatment effect heterogeneity and causal moderation in the context of psychotherapy research.

Particular attention is given to the causal framework, assumptions, estimation, and interpretation of causal moderation. An illustrative example is included to ensure an approachable format with colloquial and intuitive language with R syntax provided to encourage and ease future implementation.

This primer encourages proper consideration and interpretation of treatment effect heterogeneity and, when conditions are met, causal moderation. This knowledge improves the understanding of treatment efficacy across participant characteristics and study contexts and, relatedly, the generalizability of treatment effects.