1. Zhou Chi is at the Institute of Social and Family Medicine, Zhejiang University, Hangzhou 310058, China.
2. Therese Hesketh is at the Centre for International Health and Development, University College London, London WC1N 1EH.
3. Zhou Xu Dong is at the Institute of Social and Family Medicine, Zhejiang University, Hangzhou 310058, China.
4. Wang Xiao Lei is at the Institute of Social and Family Medicine, Zhejiang University, Hangzhou 310058, China and is also at the College of Nursing, Hangzhou Normal University, Hangzhou 310036, China.
5. Zheng Wei Jun is at the Institute of Social and Family Medicine, Zhejiang University, Hangzhou 310058, China.
6. Li Lu is at the Institute of Social and Family Medicine, Zhejiang University, Hangzhou 310058, China.