[1] Univ Washington, Inst Hlth Metr & Evaluat, Seattle, WA 98195 USA
[2] Seattle Childrens Hosp, Dept Anesthesiol & Pain Med, Seattle, WA USA
[3] Kings Coll London, Div Populat & Patient Hlth, Inst Dent, London WC2R 2LS, England
[4] Queen Mary Univ London, Inst Dent, Barts & London Sch Med & Dent, London, England