1. Cluster for Health Services Research, Norwegian Institute of Public Health , PO Box 222 Skøyen, Oslo N-0213, Norway
2. Division of Surgery, Akershus University Hospital , Lørenskog, Oslo 1478, Norway
3. Institute of Cliniacal Medicine, University of Oslo , Problemveien 7, Oslo 0315, Norway
4. Department of Quality and Research, University Hospital North Norway , Sykehusvegen 38, Tromsø 9019, Norway
5. Department of Surgery, Helgelandssykehuset , Prestmarkveien 1, Sandnessjøen 8800, Norway