Some further remarks on a superiority problem in misspecified restricted singular linear models

Razzaghi (1987) examined the difference of covariance matrices of competing estimators in misspecified restricted linear models. Further, Gross, Trenkler and Liski (1998) extended Razzaghi's result by asserting his sufficient condition for nonnegative definiteness of the covariance matrix difference to be also necessary, hi this paper, when the covariance matrix of the disturbance vector is nonnegative definite, the necessary and sufficient conditions for nonnegative definiteness of the covariance matrix difference are derived. So above results are strengthened.