[1] Univ Edinburgh, Ctr Populat Hlth Sci, GP Sect, Edinburgh EH8 9DX, Midlothian, Scotland
[2] Univ Edinburgh, Dept Nursing Studies, Sch Hlth & Social Sci, Sch Med, Edinburgh EH8 9AG, Midlothian, Scotland
[3] Univ London, Inst Educ, Social Sci Res Unit, London WC1H 0AL, England
[4] Scottish Ctr Social Res, Edinburgh EH3 9AW, Midlothian, Scotland
[5] Newcastle Univ, Inst Hlth & Soc, Fac Med Sci, Newcastle Upon Tyne NE2 4HH, Tyne & Wear, England