1. Yongtao Guan is Assistant Professor, Division of Biostatistics, Yale University, New Haven, CT 06520-8034 . The author thanks the editor, the associate editor, and two referees for their helpful comments which have greatly improved this manuscript both in content and exposition. The work was supported in part by NSF grant DMS-06-03673 and a summer research grant from the School of Business at University of Miami.