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A bias bound for least squares linear regression. (English) Zbl 0824.62057
Summary: Consider a general linear model $$y= g(\alpha+ \beta {\mathbf x})+ \varepsilon$$, where the link function $$g$$ is arbitrary and unknown. The maximal component of $$(\alpha, \beta)$$ that can be identified is the direction of $$\beta$$, which measures the substitutibility of the components of $${\mathbf x}$$. If $$\zeta (\beta {\mathbf x})= E({\mathbf x}\mid \beta {\mathbf x})$$ is linear in $$\beta {\mathbf x}$$, the least squares linear regression of $$y$$ on $${\mathbf x}$$ gives a consistent estimate for the direction of $$\beta$$, despite possible nonlinearity in the link function. If $$\zeta (\beta {\mathbf x})$$ is nonlinear, the linear regression might be inconsistent for the direction of $$\beta$$.
We establish a bound for the asymptotic bias, which is determined from the nonlinearity in $$\zeta( \beta {\mathbf x})$$, and the multiple correlation coefficient $$R^ 2$$ for the least squares linear regression of $$y$$ on $${\mathbf x}$$. According to the bias bound, the linear regression is nearly consistent for the direction of $$\beta$$, despite possible nonlinearity in the link function, provided that the nonlinearity in $$\zeta (\beta {\mathbf x})$$ is small compared to $$R^ 2$$. Our measure of nonlinearity in $$\zeta (\beta {\mathbf x})$$ is analogous to the maximal curvature studied by D. R. Cox and N. J. H. Small [Biometrika 65, 263-272 (1978; Zbl 0386.62041)]. The bias bound is tight; we give the construction for the least favorable models which achieve the bias bound. The theory is applied to a special case for an illustration.

##### MSC:
 62J05 Linear regression; mixed models 62J12 Generalized linear models (logistic models)