1. Departments of Neurosurgery and
2. Alrijne Hospital, Leiden and Leiderdorp, and
3. Department of Neurosurgery, Haaglanden Medical Center, The Hague;
4. Department of Neurosurgery, OLVG, Amsterdam;
5. University Medical Center Groningen;
6. Biostatistics, Leiden University Medical Center, Leiden;
7. Spaarne Gasthuis, Haarlem/Hoofddorp, The Netherlands