On the minimax robustness against correlation and heteroscedasticity of ordinary least squares among generalized least squares estimates of regression

Summary We revisit a result according to which certain functions of covariance matrices are maximized at scalar multiples of the identity matrix. In a statistical context in which such functions measure loss, this says that the least favourable form of dependence is in fact independence, so that a procedure optimal for independent and identically distributed data can be minimax. In particular, the ordinary least squares estimate of a correctly specified regression response is minimax among generalized least squares estimates, when the maximum is taken over certain classes of error covariance structures and the loss function possesses a natural monotonicity property. In regression models whose response function is possibly misspecified, ordinary least squares is minimax if the design is uniform on its support, but this often fails otherwise. An investigation of the interplay between minimax generalized least squares procedures and minimax designs leads us to extend, to robustness against dependencies, an existing observation: that robustness against model misspecifications is increased by splitting replicates into clusters of observations at nearby locations.