1. Medical Research Council Epidemiology Resource Centre (S.M.R., L.D.M., S.R.C., N.C.H., C.R.G., H.M.I., J.B., K.M.G., C.C.), University of Southampton, Southampton SO16 6YD, United Kingdom
2. Centre for Paediatric Epidemiology and Biostatistics (C.M.L.), Institute of Child Health, University College London, WC1N 1EH London, United Kingdom