“…It is shown, both theoretically and numerically, that imputation of missing predictors may improve the efficiency. Other popular methods include a likelihood approach, inverse-probability weighting, EM and Bayesian procedures; see reviews and recent results in Horton and Kleiman (2007), Chen, Hong, and Torozzi (2008), Zhou, Wan, and Wang (2008), Qin, Zhang, and Leung (2009), and Wang and Chen (2009). Furthermore, numerical studies of Nittner (2003Nittner ( , 2004 indicate that imputation may be useful for nonparametric regression.…”