1 Department of Neurosurgery, Trial Coordination Centre, University of Groningen, University Medical Center Groningen, 9700 RB Groningen, the Netherlands. Electronic address: n.a.bakker@umcg.nl.
2 Department of Epidemiology, Trial Coordination Centre, University of Groningen, University Medical Center Groningen, 9700 RB Groningen, the Netherlands.
3 Department of Neurosurgery, Trial Coordination Centre, University of Groningen, University Medical Center Groningen, 9700 RB Groningen, the Netherlands.