“…Since then, functional linear regression model has been further extended or modified to take into account possible nonlinear relationship, some of the regression models include the functional polynomial regression model (Yao and Müller, 2010;Horváth and Reeder, 2012), functional additive regression model (Müller and Yao, 2008;Febrero-Bande and González-Manteiga, 2013;Fan and James, 2013), and nonparametric functional regression model (Ferraty and Vieu, 2006;Ferraty, Van Keilegom, and Vieu, 2010). Due to the fast development in functional regression models, it has gained an increasing popularity in various fields of application, such as atmospheric radiation (Hlubinka and Prchal, 2007), chemometrics (Frank and Friedman, 1993;Ferraty and Vieu, 2002;Burba et al, 2009;Yao and Müller, 2010), climate variation forecasting (Shang and Hyndman, 2011), demographic modeling and forecasting (Hyndman and Ullah, 2007;Hyndman and Booth, 2008;Hyndman and Shang, 2009;Chiou and Müller, 2009), earthquake modeling , gene expression (Yao et al, 2005a;Chiou and Müller, 2007), health science (Harezlak, Coull, Laird, Magari, and Christiani, 2007), linguistics (Hastie et al, 1995;Malfait and Ramsay, 2003;Aston et al, 2010), medical research (Ratcliffe et al, 2002;Yao et al, 2005b;Erbas et al, 2007), ozone level prediction (Quintela-del-Río and Francisco-Fernández, 2011), and sulfur dioxide level prediction (Fernandez de Castro et al, 2005).…”