1. Office of Biostatistics Research, National Heart Lung and Blood Institute Bethesda 20892 Maryland USA
2. Academy of Mathematics and Systems Science Chinese Academy of Sciences Beijing 100190 China
3. Department of Statistics and Actuarial Science The University of Hong Kong PokfulamHong Kong
4. Department of Biostatistics, Bioinformatics and Biomathematics Georgetown University 20057 Washington DC USA
5. Department of Statistics George Washington University 20052 Washington DC USA