- Correspondence to Professor Gonghuan Yang, Department of Epidemiology and Statistics, Institute of Basic Medical Science, Chinese Academy of Medical Science, 5# Dong Dan San Tiao, Beijing 100005, People's Republic of China;
- Received July 22, 2012
- Revised December 20, 2012
- Accepted December 21, 2012
- First published January 24, 2013.
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