“…The ABov estimator (Arellano & Bover, 1995), for example, uses the additional moment restrictions implied by the assumption that the predetermined variable has constant correlation with the unobserved effects. In general, important recent advances have been made with regard to nonlinear GMM estimation of dynamic panel data models (see, inter alia, Ahn & Schmidt, 1995, 1997Blundell & Bond, 1998;Crepon, Kramarz & Trognon, 1998;Ahn & Schmidt, 1999;Breitung & Lechner, 1999) with regard to exploiting further orthogonality conditions. For example, Ahn & Schmidt (1995) exploit quadratic orthogonality conditions implied by the lack of serial correlation and the assumption of homoscedasticity, whereas Crepon, Kramarz & Trognon (1998) focus on the additional linear conditions implied by assumption (3.1).…”