Publication Date: June, 2016
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.
Keywords: Compliers, Dierence-in-dierence test, Sample local average treatment eect (SLATE), Marginal untreated outcome test, Massachusetts health reform, Program evaluation, Selection
JEL Classification: C1, C9, H4, I13