Optimal Smoothing for a Computationallyand StatisticallyEfficient Single Index Estimator

In semiparametric models it is a common approach to under-smooth thenonparametric functions in order that estimators of the finite dimensionalparameters can achieve root-n consistency. The requirement of under-smoothingmay result as we show from inefficient estimation methods or technical difficulties.Based on local linear kernel smoother, we propose an estimation method toestimate the single-index model without under-smoothing. Under some conditions,our estimator of the single-index is asymptotically normal and most efficient in thesemi-parametric sense. Moreover, we derive higher expansions for our estimatorand use them to define an optimal bandwidth for the purposes of index estimation.As a result we obtain a practically more relevant method and we show its superiorperformance in a variety of applications.