1. a The University of Reading , Medical and Pharmaceutical Statistics Research Unit, P.O. Box 240 Earley Gate, Reading, RG6 6FN, UK
2. b Institute of Child Health, University College London Medical School , 30 Guilford Street, London, WC1N 1EH, UK
3. c London School of Hygiene and Tropical Medicine , Medical Statistics Unit, Keppel Street, London, WC1E 7HT, UK