证明了椭球分布下泰勒估计器的最优样本复杂度,与高斯情形一致。
Optimal Bounds for Tyler's M-Estimator for Elliptical Distributions
- 引入∞-扩张伪随机条件,建立新分析框架
- 在最优样本量下实现与高斯分布相同的误差界
- 首次在低样本量下保证迭代算法收敛,适合统计学习研究者
统计学中的基础问题之一是估计椭球分布的形状矩阵,该问题推广了熟悉的高斯协方差估计。对于椭球分布,泰勒提出了一种自然的M-估计器,并在渐近情形下展示了独立于底层分布的优良统计性质。数值实验表明该估计器表现优异,且其迭代过程收敛迅速。弗兰克斯与莫伊特拉首次在有限样本设置下给出了无分布假设的误差界,并对泰勒迭代过程进行了严格收敛分析,但其结果比高斯情形的样本复杂度高出$\log^{2} d$因子。本文通过构建新的分析框架,完全弥补这一差距,证明了泰勒估计器在所有椭球分布下均达到最优样本阈值与误差界,与高斯情形完全匹配。此外,我们还在更低样本阈值下恢复了算法的收敛性。方法基于弗兰克斯与莫伊特拉的算子缩放思想,引入一种新型伪随机条件——∞-扩张,并证明椭球分布在此最优样本阈值下满足该条件,进而建立了针对该条件的新缩放定理。
原文摘要 · Abstract (English)
A fundamental problem in statistics is estimating the shape matrix of an Elliptical distribution. This generalizes the familiar problem of Gaussian covariance estimation, for which the sample covariance achieves optimal estimation error. For Elliptical distributions, Tyler proposed a natural M-estimator and showed strong statistical properties in the asymptotic regime, independent of the underlying distribution. Numerical experiments show that this estimator performs very well, and that Tyler's iterative procedure converges quickly to the estimator. Franks and Moitra recently provided the first distribution-free error bounds in the finite sample setting, as well as the first rigorous convergence analysis of Tyler's iterative procedure. However, their results exceed the sample complexity of the Gaussian setting by a $\log^{2} d$ factor. We close this gap by proving optimal sample threshold and error bounds for Tyler's M-estimator for all Elliptical distributions, fully matching the Gaussian result. Moreover, we recover the algorithmic convergence even at this lower sample threshold. Our approach builds on the operator scaling connection of Franks and Moitra by introducing a novel pseudorandom condition, which we call $\infty$-expansion. We show that Elliptical distributions satisfy $\infty$-expansion at the optimal sample threshold, and then prove a novel scaling result for inputs satisfying this condition.
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