1. MRC Biostatistics Unit Hub for Trials Methodology Research Cambridge Institute of Public Health Forvie Site, Robinson Way Cambridge CB2 0SR U.K.
2. Cardiovascular Epidemiology Unit, Strangeways Research Laboratory University of Cambridge Cambridge CB1 8RN U.K.
3. Department of Biostatistics, Mailman School of Public Health Columbia University 722 West 168th Street New York NY 10032 U.S.A.