Laura Cornelsen1, Mario Mazzocchi2, Richard D Smith3. 1. Faculty of Public Health and Policy, London School of Hygiene & Tropical Medicine, 15-17 Tavistock Place, WC1H 9SH London, United Kingdom. Electronic address: laura.cornelsen@lshtm.ac.uk. 2. Department of Statistical Sciences, University of Bologna, Via Belle Arti 41, Bologna, Italy. 3. College of Medicine and Health, University of Exeter, Medical School Building, St Luke's Campus, Magdalen Road, Exeter, EX1 2LU, United Kingdom; Faculty of Public Health and Policy, London School of Hygiene & Tropical Medicine, 15-17 Tavistock Place, WC1H 9SH London, United Kingdom.