1. Academy of Mathematics and Systems Science, Chinese Academy of Sciences, Beijing, China
2. Department of Biostatistics and Epidemiology, University of Pennsylvania, Philadelphia, Penn., U.S.A.
3. Office of Biostatistics Research, National Heart, Lung and Blood Institute, Bethesda, Maryland, U.S.A.
4. Department of Statistics, George Washington University, Washington, D.C., U.S.A.
5. Eunice Kennedy Shriver National Institute of Child Health and Human Development, Bethesda, Maryland, U.S.A.