[1] Paediat Epidemiol Grp, Leeds LS2 9LN, W Yorkshire, England
[2] Leukaemia Res Fund Ctr Clin Epidemiol, Leeds LS2 9LN, W Yorkshire, England
[3] Unit Epidemiol & Hlth Serv Res, Leeds LS2 9LN, W Yorkshire, England
[4] St James Univ Hosp, Childrens Day Hosp, Leeds LS9 7TF, W Yorkshire, England