1. Department of Epidemiology and Biostatistics, School of Public Health, Peking University , Xueyuan Road, Haidian District, Beijing 100191 , China
2. Survey Research Center, Institute for Social Research, University of Michigan , 426 Thompson St, Ann Arbor, MI 48104 , USA
3. Department of Epidemiology, School of Public Health, University of Michigan , 426 Thompson St, Ann Arbor, MI 48104 , USA
4. Department of Epidemiology, School of Public Health, Johns Hopkins Bloomberg School of Public Health , 615 N. Wolfe Street, Baltimore, MD 21205 , USA