1. Cancer Research UK Centre for Epidemiology, Mathematics and Statistics, Wolfson Institute of Preventive Medicine, Charterhouse Square, London EC1M 6BQ, UK
2. Cancer Research UK Medical Statistics Group, Centre for Statistics in Medicine, Wolfson College, Linton Road, Oxford OX2 6UD, UK
3. Health Behaviour Unit, Cancer Research UK, Department of Epidemiology and Public Health, University College, London, 2–16 Torrington Place, London WC1E 6BT, UK