1. School of Statistics, Renmin University of China, Beijing 100872, China;
2. Southern China Center for Statistical Science, Department of Statistical Science, School of Mathematics, Sun Yat-Sen University, Guangzhou 510275, China;
3. Saw Swee Hock School of Public Health, National University of Singapore, 117549 Singapore;
4. Department of Statistics and Finance/International Institute of Finance, School of Management, University of Science and Technology of China, Hefei 230026, China