## The Annals of Statistics

- Ann. Statist.
- Volume 7, Number 2 (1979), 258-268.

### Asymptotic Behavior of $M$-Estimators for the Linear Model

Victor J. Yohai and Ricardo A. Maronna

#### Abstract

This paper deals with $M$-estimators for the linear model $y_i = \mathbf{x}'_i\mathbf{\theta} + u_i1 \leqslant i \leqslant n$, where the $\mathbf{x}_i$ are fixed $p$-dimensional vectors, and the $u_i$ are i.i.d. random variables with distribution $F$. The estimators considered are solutions $\hat\mathbf{\theta}$ of the equation $\sum^n_{j = 1}\psi(y_j - \mathbf{x}'_i\hat{\mathbf{\theta}})\mathbf{x_j = 0}$ for some function $\psi$. Let $\mathbf{X}$ be the matrix whose $i$th row is $\mathbf{x}'_i$. Then it is proved that $(\mathbf{\hat{\theta} - \theta)'X'X(\hat{\theta} - \theta)}$ is bounded in probability assuming that $\psi$ satisfies a set of conditions which include $\psi$ to be monotone and $X$ to have full rank. This implies that a sufficient condition for consistency is that the smallest eigenvalue of $\mathbf{X'X}$ tends to infinity. For the case in which $p = p_n \rightarrow \infty$ it is proved that $p^{-1}(\mathbf{\hat{\theta} - \theta)'X'X(\hat{\theta} - \theta)}$ is bounded in probability, assuming that $p\varepsilon\rightarrow 0$ where $\varepsilon = \max_{1\leqslant i \leqslant n}(\mathbf{x'_iX'Xx_i})$. The asymptotic normality of these estimators is proved for both the cases of $p$ fixed and $p \rightarrow\infty$. The proof of the former is an easy consequence of a result of Bickel on one-step $M$-estimators. In the case of $p\rightarrow \infty$ we assume that $\psi$ has a bounded derivative and that $p^{3/2}\varepsilon\rightarrow 0$. This improves an analogous result by Huber, who requires $p^2\varepsilon\rightarrow 0$.

#### Article information

**Source**

Ann. Statist., Volume 7, Number 2 (1979), 258-268.

**Dates**

First available in Project Euclid: 12 April 2007

**Permanent link to this document**

https://projecteuclid.org/euclid.aos/1176344610

**Digital Object Identifier**

doi:10.1214/aos/1176344610

**Mathematical Reviews number (MathSciNet)**

MR520237

**Zentralblatt MATH identifier**

0408.62027

**JSTOR**

links.jstor.org

**Subjects**

Primary: 62G35: Robustness

Secondary: 62J05: Linear regression

**Keywords**

Robust estimation linear model consistency asymptotic normality

#### Citation

Yohai, Victor J.; Maronna, Ricardo A. Asymptotic Behavior of $M$-Estimators for the Linear Model. Ann. Statist. 7 (1979), no. 2, 258--268. doi:10.1214/aos/1176344610. https://projecteuclid.org/euclid.aos/1176344610