- Correspondence to Dr Xiaohui Xu, Department of Epidemiology, College of Public Health and Health Professional and College of Medicine, University of Florida, PO Box 100231, Gainesville, FL 32610, USA;
- Received May 27, 2013
- Revised July 21, 2013
- Accepted July 29, 2013
- First published September 10, 2013.
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