- Correspondence to Dr Mark Sheehan, The Ethox Centre, Nuffield Department of Population Health, University of Oxford, Old Road Campus, Rosemary Rue Building Headington, Oxford OX3 7LG, UK;
- Received 12 December 2012
- Revised 24 July 2013
- Accepted 28 July 2013
- Published online 17 September 2013.
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