[1] Queen Mary Univ London, Barts & London Sch Med, Ctr Primary Care & Publ Hlth, Res Design Serv,Blizard Inst, London E1 2AB, England
[2] Univ Exeter, Peninsula Coll Med & Dent, Exeter, Devon, England
[3] Univ Plymouth, Peninsula Coll Med & Dent, Exeter, Devon, England
[4] Monash Univ, Alfred Hosp, Dept Epidemiol & Prevent Med, Melbourne, Vic 3181, Australia
[5] Warwick Med Sch, Clin Trials Unit, Coventry, W Midlands, England