Universal Domination and Stochastic Domination: $U$-Admissibility and $U$- Inadmissibility of the Least Squares Estimator
Brown, Lawrence D. ; Hwang, Jiunn T.
Ann. Statist., Tome 17 (1989) no. 1, p. 252-267 / Harvested from Project Euclid
Assume the standard linear model $X_{n \times 1} = A_{n \times p} \theta_{p \times 1} + \varepsilon_{n \times 1},$ where $\varepsilon$ has an $n$-variate normal distribution with zero mean vector and identity covariance matrix. The least squares estimator for the coefficient $\theta$ is $\hat{\theta} \equiv (A'A)^{-1}A'X$. It is well known that $\hat{\theta}$ is dominated by James-Stein type estimators under the sum of squared error loss $|\theta - \hat{\theta}|^2$ when $p \geq 3$. In this article we discuss the possibility of improving upon $\hat{\theta}$, simultaneously under the "universal" class of losses: $\{L(|\theta - \hat{\theta}|): L(\cdot) \text{any nondecreasing function}\}.$ An estimator that can be so improved is called universally inadmissible ($U$-inadmissible). Otherwise it is called $U$-admissible. We prove that $\hat{\theta}$ is $U$-admissible for any $p$ when $A'A = I$. Furthermore, if $A'A \neq I$, then $\hat{\theta}$ is $U$-inadmissible if $p$ is "large enough." In a special case, $p \geq 4$ is large enough. The results are surprising. Implications are discussed.
Publié le : 1989-03-14
Classification:  Decision theory under a broad class of loss functions,  James-Stein positive part estimator,  admissibility,  62C05,  62F11,  62J07
@article{1176347014,
     author = {Brown, Lawrence D. and Hwang, Jiunn T.},
     title = {Universal Domination and Stochastic Domination: $U$-Admissibility and $U$- Inadmissibility of the Least Squares Estimator},
     journal = {Ann. Statist.},
     volume = {17},
     number = {1},
     year = {1989},
     pages = { 252-267},
     language = {en},
     url = {http://dml.mathdoc.fr/item/1176347014}
}
Brown, Lawrence D.; Hwang, Jiunn T. Universal Domination and Stochastic Domination: $U$-Admissibility and $U$- Inadmissibility of the Least Squares Estimator. Ann. Statist., Tome 17 (1989) no. 1, pp.  252-267. http://gdmltest.u-ga.fr/item/1176347014/