## Direct estimation of the index coefficient in a single-index model.(English)Zbl 1012.62043

Summary: Single-index modeling is widely applied in, for example, econometric studies as a compromise between too restrictive parametric models and flexible but hardly estimable purely nonparametric models. By such modeling the statistical analysis usually focuses on estimating the index coefficients. The average derivative estimator (ADE) of the index vector is based on the fact that the average gradient of a single index function $$f(x^{\top}\beta)$$ is proportional to the index vector $$\beta$$. Unfortunately, a straightforward application of this idea meets the so-called “curse of dimensionality” problem if the dimensionality d of the model is larger than 2. However, prior information about the vector $$\beta$$ can be used for improving the quality of gradient estimation by extending the weighing kernel in a direction of small directional derivatives. The method proposed in this paper consists of such iterative improvements of the original ADE. The whole procedure requires at most $$2 \log n$$ iterations and the resulting estimator is $$\sqrt{n}$$-consistent under relatively mild assumptions on the model independently of the dimensionality d.

### MSC:

 62G08 Nonparametric regression and quantile regression 62H12 Estimation in multivariate analysis 62G05 Nonparametric estimation 62G20 Asymptotic properties of nonparametric inference
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