Affiliation: | 1. Department of Clinical Epidemiology and Social Medicine, St. Thomas''s Hospital Medical School, London S.E.1, United Kingdon.;2. Department of Child Health, St. Mary''s Hospital, Whitworth Park, Manchester M13 0JH, United Kingdom.;3. M.R.C./D.H.S.S. Epidemiology and Medical Care Unit, Northwick Park Hospital, Harrow HA1 3UJ, United Kingdom. |