Affiliation: | 1. Department of Clinical Epidemiology and Social Medicine , St. Thomas’s Hospital Medical School , London, S.E. 1;2. Department of Medical Statistics and Epidemiology , London School of Hygiene and Tropical Medicine , Keppel St., London, W.C. 1;3. Department of Clinical Epidemiology and Social Medicine , St. Thomas’s Hospital Medical School , London, S.E. 1 |