Improved methods for causal inference and data combination

In this dissertation, we develop improved estimation of average treatment effect on the treatment (ATT) which achieves double robustness, local efficiency, intrinsic efficiency and sample boundedness, using a calibrated likelihood approach. Moreover, we consider an extension of two-group causal inference problem to a general data combination problem, and develop estimators achieving desirable properties beyond double robustness and local efficiency. The proposed methods are shown, both theoretically and numerically, to be superior in robustness, efficiency or both to various existing estimators. In the first part, we review existing estimators on average treatment effect (ATE), mainly based on Tan (2006, 2010). This review provides a useful basis for improved estimation of average treatment effect on the treated (ATT). In the second part, we propose new methods to estimate the average treatment effect on the treated (ATT), which is of extensive interest in Econometrics, Biostatistics and other research fields. This problem seems to be often treated as a simple modification or extension of that of estimating overall average treatment effects (ATE). But the propensity score is no longer ancillary for estimation of ATT, in contrast with estimation of ATE. We study the efficient influence function and the corresponding semiparametric variance bound for the estimation of ATT under three different assumptions: a nonparametric model, a correct propensity score model and known propensity score. Then we construct Augmented Inverse Probability Weighted (AIPW) estimators which are locally efficient and doubly robust. Furthermore, we develop calibrated regression and likelihood estimators that are not only doubly robust and locally efficient, but also intrinsically e cient and sample bounded. Two simulations and real data analysis on a job training program are provided to demonstrate the advantage of our estimators compared with existing estimators. In the third part, we extend our methods to a general data combination problem for moment restriction models (Chen et al. 2008). Similarly, we derive augmented inverse probability weighted (AIPW) estimators that are locally efficient and doubly robust. Moreover, we develop calibrated regression and likelihood estimators which achieve double robustness, local efficiency and intrinsic efficiency. For illustration, we take the linear two-sample instrumental variable problem as an example, and derive all the relevant estimators by applying the general estimators in this specific example. Finally, a simulation study and an Econometric application on a public housing project are provided to demonstrate the superior performance of our improved estimators.Ph.D.Includes bibliographical referencesby Heng Sh