- Correspondence to Dr Li Ling, Sun Yat-sen Center for Migrant Health Policy, School of Public Health, Sun Yat-sen University, #74, Zhongshan Road II, Guangzhou 510080, People's Republic of China;
- Received July 17, 2015
- Revised December 16, 2015
- Accepted December 23, 2015
- First published February 3, 2016.
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