Heterogeneous Causal Mediation Analysis Using Bayesian Additive Regression Trees

Music City Center 

Causal mediation analysis provides insights into the mechanisms through which treatments affect outcomes. While mediation effects often vary across individuals, most existing methods focus solely on population-average effects, overlooking individual-level heterogeneity. To address this limitation, we propose a Bayesian regression tree ensemble method that flexibly models non-linear relationships and captures treatment-by-mediator interactions in the mediation process. Using hierarchical posterior sampling, our approach provides credible intervals with nominal coverage rates for testing heterogeneous mediation effects. Additionally, we leverage regression tree summaries to identify subgroups with distinct mediation effects and employ SHapley Additive exPlanation (SHAP) values to highlight key moderators and their influence on the mediation process. Comprehensive simulations demonstrate the method's accuracy in estimating and inferring heterogeneous mediation effects. Finally, we apply our method to investigate the heterogeneous mediation of Alzheimer's disease pathology burden on the effect of apolipoprotein E (APOE) genotype on late-life cognition.

Causal Mediation Analysis; Bayesian Tree Ensembles; Heterogeneous Effects; Non-linear Interactions; Moderation Mechanisms.