[1] Gen Infirm, Ctr Digest Dis, Leeds LS1 3EX, W Yorkshire, England
[2] Univ Leeds, Paediat Epidemiol Grp, Leeds, W Yorkshire, England
[3] Univ Leeds, No & Yorkshire Clin Trials & Res Unit, Leeds, W Yorkshire, England
[4] Univ Birmingham, Dept Epidemiol & Publ Hlth, Birmingham, W Midlands, England
[5] Univ Leeds, Inst Epidemiol & Hlth Serv Res, Leeds, W Yorkshire, England
[6] Wellington Sch Med, Dept Gen Practice, Wellington, New Zealand
[7] Univ Leeds, Canc Epidemiol Grp, Epidemiol & Hlth Serv Res Unit, Leeds, W Yorkshire, England