Affiliation: | 1 Biostatistics Research Branch, National Institute of Allergy and Infectious Diseases, 6700B Rockledge Drive, MSC 7609, Bethesda, Maryland 20892, U.S.A. jingqin{at}niaid.nih.gov, 2 Department of Mathematics, University of Toledo, Toledo, Ohio 43606, U.S.A. bzhang{at}utnet.utoledo.edu |