1. Norwich Medical School, Faculty of Medicine and Health Sciences, University of East Anglia, Norwich, Norfolk NR4 7TJ, UK
2. Clinical Gerontology Unit, School of Clinical Medicine, University of Cambridge, Cambridge, UK
3. Health Policy Unit, Department of Public Health and Policy, London School of Hygiene and Tropical Medicine, London, UK
4. Strangeway Research Laboratory, Department of Public Health and Primary Care, University of Cambridge, Cambridge, UK
5. MRC Epidemiology Unit, Cambridge, UK