- Correspondence to Professor Dongfeng Zhang, Department of Epidemiology and Health Statistics, The Medical College of Qingdao University, 38 Dengzhou Road, Qingdao, Shandong 266021, People's Republic of China;
- Received June 27, 2015
- Revised July 29, 2015
- Accepted August 3, 2015
- First published September 10, 2015.
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