1. College of Applied Sciences, Beijing University of Technology, Beijing, China
2. Department of Statistics, University of Illinois at Urbana-Champaign, Il., U.S.A.
3. Department of Statistics, University of Michigan, Ann Arbor, Mi., U.S.A.
4. School of Statistics, Renmin University of China, Beijing, China
5. West China Hospital, Chengdu, China; and Department of Biostatistics, University of Michigan, Ann Arbor, Mi., U.S.A.