Doing More When You're Running LATE: Applying Marginal Treatment Effect Methods to Examine Treatment Effect Heterogeneity in Experiments

I examine treatment effect heterogeneity within an experiment to inform external validity.The local average treatment effect (LATE) gives an average treatment effect for compliers.I bound and estimate average treatment effects for always takers and never takers by extending marginal treatment effect methods.I use these methods to separate selection from treatment effect heterogeneity, generalizing the comparison of OLS to LATE.Applying these methods to the Oregon Health Insurance Experiment, I find that the treatment effect of insurance on emergency room utilization decreases from always takers to compliers to never takers.Previous utilization explains a large share of the treatment effect heterogeneity.Extrapolations show that other expansions could increase or decrease utilization.