The Effect of Medicaid Expansion on Wait Time in the Emergency Department

We study the effect of Medicaid expansion on wait time in the emergency department (ED), using a difference-in-differences approach, where the treatment group includes the states that expanded Medicaid at the beginning of 2014 and the control group includes the states that did not expand Medicaid before 2016. We first focus on the average treatment effect and find that Medicaid expansion increases ED wait time by 10.4% (or 3.5 minutes). We then develop a first-difference causal forest (FDCF) approach for heterogeneous treatment-effect analysis by incorporating the first-difference approach into the causal forest approach. Our comprehensive simulation studies show that the FDCF approach has smaller mean-squared errors than direct applications of the causal forest approach. Finally, we apply the FDCF approach to our empirical example and find that the effect of Medicaid expansion varies widely across different hospitals. Our results are useful to policymakers deciding whether to expand Medicaid and to hospital managers seeking to understand the effect of Medicaid expansion on their hospitals. Our results are also useful to researchers applying causal machine learning methods for heterogeneous treatment-effect analysis.