1. Methodology and Statistics, Psychology, Open Universiteit, 6401 DL Heerlen, Netherlands
2. CAPHRI School for Public Health and Primary Care, Maastricht University, PO Box 616, 6200 MD Maastricht, The Netherlands
3. Department of Epidemiology and Biostatistics, Amsterdam University Medical Centres, PO Box 7057, 1007 MB Amsterdam, The Netherlands