“…Roughly speaking, a model with a larger dimension has a smaller approximation error but a larger estimation error, whereas a model with a smaller dimension has a smaller estimation error but a larger approximation error. The problem of model selection is to empirically select the finite-dimensional parametric model (from the permissible collection of models {S m } m ∈ M n ) that achieves the best tradeoff between the competing approximation error and estimation error components-and, consequently, achieves the smallest possible statistical risk in estimating s. For previous theoretical work on model selection in the context of nonparametric regression estimation, see, for example, Barron (1991Barron ( , 1994, Barron, Birgé, & Massart (1996), Birgé & Massart (1994b), Baum & Haussler (1989), Haussler (1992), Lugosi & Nobel (1995), Lugosi & Zeger (1996), McCaffrey & Gallant (1994), Modha & Masry (1996, 1998, Rissanen (1989), Shen & Wong (1994), Vapnik (1982Vapnik ( , 1995, and White (1989).…”