1. Department of Statistics, University of Oxford, 1 South Parks Road, Oxford OX1 3TG, U.K.
2. The Broad Institute of MIT and Harvard, 1 Kendall Square, Building 300, Cambridge, MA 02139-1561, U.S.A.
3. Department of Epidemiology and Public Health, Imperial College, St. Mary's Campus, Norfolk Place, London W2 1PG, U.K.