A large part of the recent literature on program evaluation has focused on estimation of theaverage effect of the treatment under assumptions of unconfoundedness or ignorability following the seminal work by Rubin (1974) and Rosenbaum and Rubin (1983). In many cases however, researchers are...

Properties of GMM estimators are sensitive to the choice of instruments. Usingmany instruments leads to high asymptotic asymptotic efficiency but can causehigh bias and/or variance in small samples. In this paper we develop and implementasymptotic mean square error (MSE) based criteria for...