Qiguang, Wu, Existence of the Uniformly Minimum Risk Unbiased Estimator in Seemingly Unrelated Regression System, Acta Mathematica Sinica, New Series Chinese Journal of Mathematics, 11(1), 1995, pp. 23-28
For a seemingly unrelated regression system with the assumption of normality, a necessary and sufficient condition for the existence of the Uniformly Minimum Risk Unbiased (UMRU) estimator of regression coefficients under strictly convex loss is obtained; it is proved that any unbiased estimator can not improve the least squares estimator; it is also shown that no UMRU estimator exists under missing observations.