This
paper is an extension of Magnus (2002) to multiple dimensions. We consider estimation
of a multivariate normal mean under sum of squared error loss. We construct the
efficiency bound (the lowest achievable risk) for minimax shrinkage estimators
satisfying the sufficient conditions of Efron and
Morris (1976). This allows us to compare the regret of existing shrinkage
estimators. We also construct a new shrinkage estimator which achieves
substantially lower maximum regret than existing estimators.
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Some
of the above material is based upon work supported by the National Science
Foundation under Grant No. SES-0550908. Any opinions, findings, and
conclusions, or recommendations expressed in this material are those of the
author(s), and do not necessarily reflect the views of the NSF.