1. University Neurosurgical Center Holland, Leiden University Medical Center, Haaglanden Medical Center and Haga Teaching Hospital, Leiden-The Hague, Department of Neurosurgery, Leiden, The Netherlands.
2. Department of Clinical Epidemiology, Leiden University Medical Center, Leiden, The Netherlands.
3. Directorate of Research Policy (Walaeus Library), and Leiden University Medical Center, Leiden, The Netherlands.
4. Department of Neurosurgery, Antwerp University Hospital and University of Antwerp, Edegem, Belgium.