[1] Univ Manchester, Res Sch Translat Med, Epidemiol Res Grp, Epidemiol Unit,ARC, Manchester M13 9PT, Lancs, England
[2] Univ Manchester, Natl Primary Care Res & Dev Ctr, Manchester M13 9PT, Lancs, England
[3] Univ Manchester, Psychiat Grp, Manchester M13 9PT, Lancs, England
[4] Univ Manchester, Hlth Methodol Res Grp, Sch Community Based Med, Manchester M13 9PT, Lancs, England
[5] Univ Aberdeen, Sch Med, Dept Publ Hlth, Aberdeen Pain Res Collaborat,Epidemiol Grp, Aberdeen AB9 2ZD, Scotland