1. Miguel A. Fernández , Cristina Rueda are Associate Professors, and Bonifacio Salvador is Professor, Department of Statistics and O. R., Facultad de Ciencias, Universidad de Valladolid, 47005 Valladolid, Spain. This research was supported in part by Spanish DGES grant MTM2004-07740 and by PAPIJCL grant VA047A05. The authors thank the referees, the associate editor, and the editor for useful comments that led to improvements in the presentation of the article.