Medical Research Council Biostatistics Unit, Institute of Public Health, Cambridge, CB2 0SR, UK, JDRF/Wellcome Trust Diabetes and Inflammation Laboratory, NIHR Cambridge Biomedical Research Centre, Cambridge Institute for Medical Research, University of Cambridge, Addenbrookes Hospital, Cambridge, CB2 0XY, UK and Faculty of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, London, WC1E 7HT, UK.