- Correspondence to Dr Li Liu, Department of Epidemiology and Biostatistics, and the Ministry of Education Key Lab of Environment and Health, School of Public Health, Tongji Medical College, Huazhong University of Science and Technology, No.13 of Hangkong Road, Wuhan, Hubei 430030, China
- Accepted 1 September 2015
- Published in print 1 March 2016.
- Published online 2 March 2016.
- Published first 18 September 2015.
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