[1] Linkoping Univ, Dept Med & Hlth Sci, Linkoping, Sweden
[2] Linkoping Univ, Dept Comp & Informat Sci, S-58183 Linkoping, Sweden
[3] Cambridge Inst Publ Hlth, MRC Biostat Unit, Cambridge, England
[4] Univ York, Dept Hlth Sci, Heslington, England
[5] Linkoping Univ, Dept Med Specialist, Campus US, SE-58183 Motala, Sweden
[6] Linkoping Univ, Dept Med & Hlth Sci, Campus US, SE-58183 Motala, Sweden