1. Department of Research, Stavanger University Hospital, N-4068 Stavanger, Norway
2. Department of Neurosurgery, Stavanger University Hospital, N-4068 Stavanger, Norway
3. Department of Research, Section of Biostatistics, Stavanger University Hospital, N-4068 Stavanger, Norway
4. Network for Medical Sciences, University of Stavanger, 4036 Stavanger, Norway