Before revision:  We derive asymptotic results for semiparametric and parametric estimators for repeated cross-sections and panel data and show  desirable statistical properties . Notably, a non-standard semiparametric efficiency bound for  difference-in-differences estimation that incorporates the repeated cross-section case is established. Our proposed semiparametric estimator is shown to attain this bound.
After revision:  We derive asymptotic results for semiparametric and parametric estimators for repeated cross-sections and panel data and show desirable statistical properties . Notably, a non-standard semiparametric efficiency bound for difference-in-differences estimation . The proposed semiparametric estimator is shown to attain this bound.