1. M Ketelaar, PhD, is Research Coordinator, Rehabilitation Center De Hoogstraat, Rembrandtkade 10, 3583 TM Utrecht, the Netherlands.
2. A Vermeer, PhD, is Professor, Department of Educational Sciences, Faculty of Social Sciences, Utrecht University, Utrecht, the Netherlands
3. H't Hart, PhD, is Professor, Department of Methodology and Statistics, Faculty of Social Sciences, Utrecht University
4. E van Petegem-van Beek, PT, MSc, is Research Assistant, Department of Educational Sciences, Faculty of Social Sciences, Utrecht University
5. PJM Helders, PhD, is Professor, Department of Pediatrics, Faculty of Medicine, University Medical Center Utrecht, and University Children's Hospital, Wilhelmina Kinderziekenhuis, Utrecht, the Netherlands