1. Department of Obstetrics and Gynecology, Akershus University Hospital, P.O. Box 1000, N-1478 Lørenskog, Norway
2. Institute of Clinical Medicine, University of Oslo, P.O. Box 1171, Blindern, N-0318 Oslo, Norway
3. Department of Mammography Screening, Cancer Registry of Norway, P.O. Box 5313, Majorstuen, N-0304 Oslo, Norway
4. Oslo Centre for Biostatistics and Epidemiology, Department of Biostatistics, University of Oslo, P.O. Box 1122, Blindern, 0372 Oslo, Norway