[1] Imperial Coll London, Dept Epidemiol & Biostat, London, England
[2] Addenbrookes Hosp, MRC, Epidemiol Unit, Cambridge, England
[3] Univ Med Ctr Utrecht, Julius Ctr Hlth Sci & Primary Care, Utrecht, Netherlands
[4] Karolinska Inst, Dept Med Epidemiol & Biosttist, S-10401 Stockholm, Sweden