[1] Univ Oxford, Med Res Council, Populat Hlth Res Unit, Oxford OX3 7LF, England
[2] Univ Oxford, Nuffield Dept Populat Hlth, Clin Trial Serv Unit, Roosevelt Dr, Oxford OX3 7LF, England
[3] Univ Oxford, Nuffield Dept Populat Hlth, Epidemiol Studies Unit CTSU, Roosevelt Dr, Oxford OX3 7LF, England
[4] Oxford Univ Hosp, Oxford Biomed Res Ctr, Natl Inst Hlth Res, Old Rd, Oxford OX3 7LE, England
[5] Univ Bristol, Med Res Council Integrat Epidemiol Unit, Oakfield House, Bristol BS8 2BN, Avon, England
[6] MRC, Integrat Epidemiol Unit, Oakfield House, Bristol BS8 2BN, Avon, England
[7] Natl Inst Hlth Res, Bristol Biomed Res Ctr, Oakfield House, Bristol BS8 2BN, Avon, England
[8] Univ Bristol, Sch Social & Community Med, Barley House, Bristol BS8 2BN, Avon, England