1. School of Statistics; Renmin University of China; Beijing 100872 China
2. School of Public Health; Yale University; New Haven CT 06511 USA
3. Center for Applied Statistics; Renmin University of China; Beijing 100872 China
4. International College (Suzhou Research Institute); Renmin University of China; Suzhou Jiangsu 215123 China
5. Department of Scientific Research; Shandong University of Traditional Chinese Medicine; Jinan Shandong 250355 China
6. Department of Applied Mathematics and Statistics; Johns Hopkins University; Baltimore MD 21218 USA