- Correspondence to Dr Qing-Bao Tian, Department of Epidemiology and Statistics, School of Public Health, Hebei Medical University, 361 East Zhongshan Road, Shijiazhuang 050000, Hebei, China;
- Received August 10, 2015
- Revised October 12, 2015
- Accepted November 7, 2015
- First published November 26, 2015.
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