1. MRC Biostatistics Unit, Institute of Public Health, Robinson Way, Cambridge CB2 0SR, U.K.
2. Medical and Pharmaceutical Statistics Research Unit, Department of Mathematics and Statistics, Lancaster University, Lancaster LA1 4YF, U.K.
3. Wolfson Institute of Preventive Medicine, Bart's and The London School of Medicine and Dentistry, Queen Mary University of London, London EC1M 6BQ, U.K.