1. From the Clinical Epidemiology Group (K.C., L.R., S.G.A.) and the Biostatistics Group (G.D.), School of Epidemiology & Health Sciences, University of Manchester Medical School, Manchester, UK; Department of Medicine (J.S.W.), Northwick Park Hospital and Clinical Research Centre, Harrow, UK; and Emeritus Professor (R.G.G.), Department of Ultrasonic Angiology, Guy’s Hospital, University of London, London, UK.