Abstract
There are two classes of estimators for the error variance in nonparametric regression: residual-based estimators and difference-based estimators. Residual-based estimators require an estimator of the regression function and are asymptotically equivalent to the sample variance based on the actual errors. Difference-based estimators avoid estimating the regression function and are thus simpler to calculate. They also possess superior bias properties at the expense of larger variances. Müller et al. [U.U. Müller, A. Schick, and W. Wefelmeyer, Estimating the error variance in nonparametric regression by a covariate-matched U-statistics, Statistics 37 (2003), pp. 179–188.] suggested improving difference-based estimators using covariate matching. They showed that a covariate-matched version of Rice's [J. Rice, Bandwidth choice for nonparametric regression, Ann. Statist. 12 (1984), pp. 1215–1230.] difference-based estimator matches the asymptotic performance of residual-based estimators, yet still possesses the good bias properties of Rice's estimator. Here we prove a similar result for a covariate-matched version of the difference-based estimator of Gasser et al. [T. Gasser, L. Sroka, and C. Jennen-Steinmetz, Residual variance and residual pattern in nonlinear regression, Biometrika 73 (1986), pp. 625–633.] as their estimator has even better bias properties than Rice's estimator.
Acknowledgements
We thank the Associate Editor and two anonymous reviewers for thoughtful comments leading us to add Remarks 1.2 and 1.3 and to develop a data-driven bandwidth selection method. A. Schick was supported in part by NSF Grant DMS 0405791.