[1] MRC, Biostat Unit, Cambridge, England
[2] Univ Strathclyde, Dept Math & Stat, Glasgow, Lanark, Scotland
[3] NHS Hlth Scotland, Publ Hlth Sci Directorate, Glasgow, Lanark, Scotland
[4] Glasgow Caledonian Univ, Sch Hlth & Life Sci, Inst Appl Hlth Res, Glasgow, Lanark, Scotland
[5] Western Infirm & Associated Hosp, Emergency Dept, Glasgow, Lanark, Scotland
[6] Addict Serv NHS Greater Glasgow & Clyde, Possilpark Hlth & Care Ctr, Glasgow, Lanark, Scotland