1. Department of General Practice, Institute of Health and Society, University of Oslo , P.O. Box 1130, Blindern, NO-0318 Oslo , Norway
2. Unicare Jeløy , Moss , Norway
3. Oslo Centre for Biostatistics and Epidemiology, Department of Biostatistics, Institute of Basic Medical Sciences, University of Oslo , Oslo , Norway
4. Department of Interdisciplinary Health Sciences, Institute of Health and Society, University of Oslo , Oslo , Norway
5. Independent Researcher , Vestre Sandøya , Norway