- Correspondence to Dr Rob J Hallifax, Oxford Respiratory Trials Unit, University of Oxford, Churchill Hospital, Oxford OX3 7LJ, UK;
- Received 21 October 2015
- Revised 2 August 2016
- Accepted 15 August 2016
- Published in print 1 December 2017.
- Published online 15 November 2017.
- Published first 1 November 2016.
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- Data supplement 1 - Online appendix
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