1. Jeng-Min Chiou is Associate Research Fellow, Institute of Statistical Science, Academia Sinica, Taipei 11529, Taiwan . Pai-Ling Li is Assistant Professor, Department of Statistics, Tamkang University, Taipei County 25137, Taiwan . We are very grateful to the associate editor and two anonymous referees for many helpful suggestions that substantially improved the article. This research was supported in part by grants from the National Science Council (NSC95-2118M001-013MY3).