[1] Univ Exeter, Exeter EX2 4TE, Devon, England
[2] Univ Edinburgh, Ctr Populat Hlth Sci, Edinburgh EH8 9AG, Midlothian, Scotland
[3] Univ Cambridge, Cambridge Ctr Hlth Serv Res, Dept Publ Hlth & Primary Care, Cambridge CB2 0SR, England
[4] UCL, Dept Epidemiol & Publ Hlth, London WC1E 7HB, England