https://docs.google.com/spreadsheets/d/13iyiJS7PY4zhF5cU9JpoZXIqMpIyoAyY3pEh8d7pnRM/edit?usp=sharing
Abstract
I examine treatment effect heterogeneity within an experiment to inform external validity. The local average treatment effect (LATE) gives and 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.