Estimating Causal Impacts of Scaling a Voluntary Policy Intervention

Evaluations often inform future program implementation decisions. However, the implementation context may differ, sometimes substantially, from the evaluation study context. This difference leads to uncertainty regarding the relevance of evaluation findings to future decisions. Voluntary interventions pose another challenge to generalizability, as we do not know precisely who will volunteer for the intervention in the future. We present a novel approach for estimating target population average treatment effects among the treated by generalizing results from an observational study to projected volunteers within the target population (the treated group). Our estimation approach can accommodate flexible outcome regression estimators such as Bayesian Additive Regression Trees (BART) and Bayesian Causal Forests (BCF). Our generalizability approach incorporates uncertainty regarding target population treatment status into the posterior credible intervals to better reflect the uncertainty of scaling a voluntary intervention. In a simulation based on real data, we demonstrate that these flexible estimators (BCF and BART) improve performance over estimators that rely on parametric regressions. We use our approach to estimate impacts of scaling up Comprehensive Primary Care Plus, a health care payment model intended to improve quality and efficiency of primary care, and we demonstrate the promise of scaling to a targeted subgroup of practices.