- 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; or
- Received July 16, 2013
- Revised September 22, 2013
- Accepted September 25, 2013
- First published October 16, 2013.
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