1. School of Mathematics and Southern China Center for Statistical Science; Sun Yat-sen University; Guangzhou Guangdong 510275 China
2. LMAM, School of Mathematical Sciences; Peking University; Beijing 100871 China
3. Zhongshan School of Medicine; Sun Yat-sen University; Guangzhou Guangdong 510080 China
4. SYSU-CMU Shunde International Joint Research Institute; Shunde Guangdong 528300 China
5. Department of Biostatistics; Yale University School of Public Health; New Haven Connecticut 06511 U.S.A.