NOTE: The X'X matrix has been found to be singular, and a generalized inverse was used to solve the normal equations. Terms whose estimates are followed by the letter 'B' are not uniquely estimable.

Yanqing Hu, Feifang Hu, ASYMPTOTIC PROPERTIES OF COVARIATE-ADAPTIVE RANDOMIZATION, The Annals of Statistics, Vol. 40, No. 3 (June 2012), pp. 1794-1815 ...