[1] Karolinska Inst, Child & Adolescent Publ Hlth Epidemiol Grp, Dept Publ Hlth Sci, Epidemiol Res Unit, SE-17176 Stockholm, Sweden
[2] Sundsvall Hosp, Dept Publ Hlth & Res, Sundsvall, Sweden
[3] Swedish Natl Board Hlth & Welfare, Ctr Epidemiol, Stockholm, Sweden