Identifying Treatment Effects in the Presence of Confounded Types
53 Pages Posted: 6 Nov 2017 Last revised: 15 Feb 2019
Date Written: November 2, 2017
In this paper, I consider identification of treatment effects when the treatment is endogenous. The use of instrumental variables is a popular solution to deal with endogeneity, but this may give misleading answers when the instrument is invalid. I show that when the instrument is invalid due to correlation with the first stage unobserved heterogeneity, a second (also possibly invalid) instrument helps partially identify not only the local average treatment effect but also the entire potential outcomes distributions for compliers. I exploit the fact that the distribution of the observed outcome in each group defined by the treatment and the instrument is a mixture of the distributions of interest. I write the identified set in the form of conditional moment inequalities, and provide an easily implementable inference procedure. Under some (testable) tail restrictions, the potential outcomes distributions are point-identified for compliers. Finally, I illustrate my methodology on data from the National Longitudinal Survey of Young Men to estimate returns to college using college proximity as (potential) instrument. I find that a college degree increases the average hourly wage of the compliers by 38-79%.
Keywords: Potential outcome, instrumental variable, LATE, compliers, mixture models
JEL Classification: C16, C21, C25, C26
Suggested Citation: Suggested Citation