[1] Univ Southampton, Div Primary Care & Populat Hlth, Southampton, Hants, England
[2] Univ Cambridge, Dept Publ Hlth & Primary Care, Cambridge, England
[3] Univ Oxford, Nuffield Dept Clin Neurosci, Oxford, England
[4] Univ Oxford, Nuffield Dept Primary Care Hlth Sci, Oxford, England
[5] Univ Bristol, Populat Hlth Sci, Bristol, Avon, England
[6] Univ Nottingham, Sch Med, Div Primary Care, Nottingham, England
[7] Univ Oxford, Clin Trial Serv Unit, Nuffield Dept Populat Hlth, Oxford, England
[8] Univ Bristol, Natl Inst Hlth Res, Bristol Biomed Res Ctr, Bristol, Avon, England