Title: Propensity-Score-Based Methods versus MTE-Based Methods in Causal Inference
Advisor: Professor Yu Xie
Committee Members: Professor Kerby Shedden, Professor Susan Murphy
Abstract: Since the seminal introduction of the propensity score by Rosenbaum and Rubin, propensityscore-based (PS-based) methods have been widely used for drawing causal inferences in the behavioral and social sciences. However, the propensity score approach depends on the ignorability assumption: there are no unobserved confounders once observed covariates are taken into account. For cases where this assumption may be violated, Heckman and his associates have recently developed a novel approach based on marginal treatment effects (MTE), which, however, has not been widely used in substantive research. In this presentation, we first explicate consequences for PS-based methods when aspects of the ignorability assumption are violated, and then compare PS-based methods and MTE-based methods by a close examination of their identification assumptions and estimation performances. Finally, we illustrate these two methods in quantifying the economic return to college using data from NLSY 1979, and discuss discrepancies in their estimation results.