Department of Mathematics, Brunel University London.|Department of Epidemiology and Biostatistics, Imperial College London, London.|Cambridge Institute for Medical Research, University of Cambridge, Cambridge, MRC Biostatistics Unit, Cambridge Institute of Public Health, Cambridge.|MRC Clinical Sciences Centre, Imperial College London, London, UK.|Cambridge Institute for Medical Research, University of Cambridge, Cambridge.|Cambridge Institute for Medical Research, University of Cambridge, Cambridge.|MRC Clinical Sciences Centre, Imperial College London, London, UK, Duke-NUS Graduate Medical School, Singapore, Singapore.|Department of Mathematics, Imperial College London, London, UK and Department of Medical Genetics, University of Cambridge.|MRC Biostatistics Unit, Cambridge Institute of Public Health, Cambridge.