[1] Univ Leeds, Gen Infirm, Ctr Digest Dis, Leeds LS1 3EX, W Yorkshire, England
[2] Univ Leeds, Paediat Epidemiol Grp, Leeds LS1 3EX, W Yorkshire, England
[3] Univ Leeds, No & Yorkshire Clin Trials & Res Unit, Leeds LS1 3EX, W Yorkshire, England
[4] Univ Leeds, Inst Epidemiol & Hlth Serv Res, Leeds LS1 3EX, W Yorkshire, England
[5] Univ York, Ctr Hlth Econ, York YO1 5DD, N Yorkshire, England
[6] Wellington Sch Med, Dept Gen Practice, Wellington, New Zealand