Article in a refereed journal:

Jean-Pierre Florens and Léopold Simar, "Parametric Approximations of Nonparametric Frontiers", Journal of Econometrics, vol. 124, n. 1, January 2005, p. 91-116.

Jean-Pierre Florens and Léopold Simar, Parametric Approximations of Nonparametric Frontiers, 2004.