1. Department of Statistics and Actuarial Science , University of Waterloo, Waterloo , Ontario , Canada N2L 3G1
2. Department of Statistical Science, School of Mathematics , Sun Yat-sen University , Guangzhou , China
3. Departments of Epidemiology and Biostatistics , Harvard School of Public Health , 677 Huntington Ave , Boston , MA 02115 , USA
4. Departments of Epidemiology and Biostatistics , Harvard School of Public Health , 677 Huntington Ave , Boston , MA 02115 , USA ; Department of Biostatistics, Yale School of Public Health, New Haven, CT 06510