1. Department of Pediatrics (P.E., J.F.L.), Örebro University Hospital, SE-701 85 Örebro, Sweden
2. Department of Clinical Epidemiology Unit (S.M.M., A.E., J.F.L.), Department of Medicine, Karolinska Institute, SE-171 77 Stockholm, Sweden
3. Department of Clinical Research Centre (S.M.M.), Örebro University Hospital, SE-701 85 Örebro, Sweden
4. Department of Medical Sciences (O.K.), Uppsala University, University Hospital, SE-751 05 Uppsala, Sweden
5. Department of Harvard School of Public Health (A.E.), Boston, Massachusetts 02115