Bias correction can often improve the finite sample performance of estimators. We show that the choice of bias correction method has no effect on the higherorder variance of semiparametrically efficient parametric estimators, so long as the estimate of the bias is asymptotically linear. It is...

Two step estimators with a nonparametric first step are important, particularly for sample selection models where the first step is estimation of the propensity score. In this paper we consider the efficiency of such estimators. We characterize the efficient moment condition for a given first...

Sample selection models are important for correcting for the effects of nonrandom sampling in microeconomic data. This note is about semiparametric estimation using a series approximation to the selection correction term. Regression spline and power series approximations are considered....