Some generalizations of Milliken's necessary and sufficient condition for estimability of linear parametric functions in linear models are established. The more universal character of the present theorems consists in avoiding the assumption of the linear independence of examined functions, in using any generalized inverse instead of the Moore-Penrose inverse, and in extending the criterion on more general linear models.
"Extensions of Milliken's Estimability Criterion." Ann. Statist. 4 (3) 639 - 641, May, 1976. https://doi.org/10.1214/aos/1176343471