1. Department of preventive medicine Institute of General Practice and Community Medicine University of Oslo POB, Blindern, Oslo, Norway, Correspondence address: Kitty Strand Department of preventive medicine Institute of General Practice and Community Medicine University of Oslo POB, 1130 Blindern, NO-0317 Oslo, Norway.
2. Department of preventive medicine Institute of General Practice and Community Medicine University of Oslo POB, Blindern, Oslo, Norway