- Correspondence to Wentian Lu, ESRC International Centre for Lifecourse Studies in Society and Health (ICLS), Research Department of Epidemiology and Public Health, Institute of Epidemiology and Healthcare, University College London, 1-19 Torrington Place, London WC1E, 7HB, UK;
- Received 2 June 2016
- Revised 11 October 2016
- Accepted 3 November 2016
- Published online 4 February 2017.
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