Stein估计量的进一步思考;题目修改为:极坐标视角下的Stein估计量
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摘要: 同时估计三个及以上同方差独立正态总体均值时, Stein23证明了最大似然估计平方损失下的不可容许性, 并同James显式构造了具有精确一致更优风险函数的压缩型估计量. 这一惊人发现—维数大于等于3时显式结构精确更优压缩型估计量, 激发了大量后续研究. Statistical Science杂志2012年组织一期专刊, “MINIMAX SHRINKAGE ESTIMATION: A TRIBUTE TO CHARLES STEIN”, 表达对Stein发现的持续赞美. James和Stein15特定变换和Stein引理24, 25是计算Stein估计量风险函数的两种基本途经. 本文基于极坐标变换, 对Stein估计量临界维数给出了解释, 并提供了其风险函数计算的备用方式. 极坐标变换既可以作为已有方法的补充, 其本身在使用Stein引理验证绝对可积性时也发挥着重要作用. 对异方差正态模型均值参数的同时估计, 文献上相对缺乏兼具显式结构和精确更优风险函数的相关研究. 本文在Stein原始估计量构成基础上, 提出了一类显式估计量, 并通过计算和观察其风险函数讨论了各待定系数的选取问题. 论文工作为进一步认识Stein发现提供了有益补充.Abstract: For simultaneously estimating three or larger mean parameters from independent normal distribution with common variance, Stein 23 proved the inadmissibility of the usual estimator, and constructed jointly with James a uniformly better estimator in the sense of mean squared error loss with closed-form expression. This astonishing discovery—better uniformly with explicit form when parameter dimension ≥ 3, inspires large amount of continuing research. Statistical Science organized a special section in 2012, “MINIMAX SHRINKAGE ESTIMATION: A TRIBUTE TO CHARLES STEIN”, to express continuing tribute to Charles Stein. The carefully designed transformation of James and Stein 15 and Stein’s basic lemma (24, 25) are two basic approaches to computing risk function of Stein’s rule. This paper provides a third way for solving this problem from a polar-coordinate’s perspective. The new perspective is a useful complement on its own right, and meanwhile plays important role for checking absolute integrability in the course of using Stein’s lemma. Besides, there are relatively few work in the literature focusing particularly on heteroscedastic normal models that are as elegant as Stein’s original work, closed-form expression as well as exact risk computation. To this end, we provide a class of estimators with explicit structure inspired by James and Stein’s original construction and find the most appropriate values of coefficients among this class by direct computing and matching. Findings in this paper provide a useful reference for further studies in this direction.