1. Faculty of Medicine, Institute of Clinical Medicine, University of Oslo and Division of Paediatric and Adolescent Medicine, Rikshospitalet, Oslo University Hospital, Oslo, Norway
2. Department of General Practice, Institute of Health and Society, University of Oslo, Oslo, Norway
3. Oslo Center for Statistics and Epidemiology, Oslo University Hospital, Oslo, Norway
4. Division of Paediatric and Adolescent Medicine, Rikshospitalet, Oslo University Hospital, Oslo, Norway