1 Institute of Epidemiology and Health Care, University College London, 1-19 Torrington Place, London, WC1E 6BT, UK. g.black@ucl.ac.uk.
2 Institute of Epidemiology and Health Care, University College London, 1-19 Torrington Place, London, WC1E 6BT, UK.
3 Wolfson Institute of Population Health, Barts and The London School of Medicine and Dentistry, Queen Mary University of London, London, EC1M 6BQ, UK.
4 Department of Public Health and Primary Care, University of Cambridge, Cambridge, CB1 8RN, UK.
5 University of Exeter Medical School, University of Exeter, Exeter, EX1 1SU, UK.
6 School of Health Sciences, University of Surrey, Guildford, GU2 7YH, UK.