Who increases emergency department use? New insights from the Oregon health insurance experiment

We provide new insights regarding the headline result that Medicaid increased emergency department (ED) use from the Oregon experiment. Using causal machine learning methods, we find meaningful heterogeneous impacts of Medicaid on ED use. The distribution of conditional treatment effects includes a wide range of negative and positive values, suggesting that the average effect masks substantial heterogeneity. A small group—about 14% of participants—in the right tail of the distribution drives the overall effect. We identify priority groups with economically significant increases in ED use based on demographics and previous utilization. Intensive margin effects are an important driver of increases in ED utilization.