1. Department of Community Medicine, Institute of Public Health, Robinson Way, Cambridge, CB2 2SR, UK
2. Cambridge and Huntingdon Breast Screening Service, Rosie Maternity Hospital, Robinson Way, Cambridge, CB2 2SW, UK
3. MRC-Biostatistics Unit, Institute of Public Health, Robinson Way, Cambridge, CB2 2SR, UK
4. Department of Clinical Gerontology, Strangeways Research Laboratory, Worts Causeway, Cambridge, CB1 4RN, UK