[1] UCL, Dept Epidemiol & Publ Hlth, Canc Res UK Hlth Behav Unit, London WC1E 6BT, England
[2] Univ Sydney, Sch Publ Hlth, Screening & Test Evaluat Programme, Sydney, NSW 2006, Australia
[3] Manchester Royal Infirm, Manchester Cytol Ctr, Manchester M13 9WW, Lancs, England
[4] Wolfson Inst Prevent Med, Canc Res UK Dept Epidemiol Math & Stat, London EC1M 6BQ, England