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For a normally distributed mixed model with two unknown variance components ₁ and ₂, a tractable characterization is given for the admissible estimators within the class N_ of invariant quadratic unbiased estimators for ₁₁ + ₂₂. Here the term admissible is used with reference only to the class N_. This characterization is based on a result for general linear models which characterizes the admissible estimators within the class of linear unbiased estimators. The admissibility of MINQUE estimators and the usual analysis of variance estimators is considered.
Olsen et al. (Wed,) studied this question.