1 Faculty of Public Health and Policy, London School of Hygiene & Tropical Medicine, London WC1H 9SH, UK. Electronic address: mark.petticrew@lshtm.ac.uk.
2 Department of Health Services Research and Policy, London School of Hygiene & Tropical Medicine, London WC1H 9SH, UK; Behaviour and Health Research Unit, Institute of Public Health, University of Cambridge, Cambridge CB2 0SR, UK.
3 Behaviour and Health Research Unit, Institute of Public Health, University of Cambridge, Cambridge CB2 0SR, UK.