1. Department of Health Policy and Management, School of Public Health, Peking University Health Science Center , No. 38 Xueyuan Road, Beijing 100191, China
2. National School of Development, Peking University , No. 5 Yiheyuan Road, Beijing 100871, China
3. Department of Global Health and Population, Harvard T.H. Chan School of Public Health , 677 Huntington Avenue, Boston, MA 02115, USA