1. Department of Epidemiology & Biostatistics, Amsterdam Public Health Research Institute, Amsterdam University Medical Centers, PO Box 7057, 1007 MB Amsterdam, The Netherlands and Mathematical Institute, Leiden University, PO Box 9512, 2300 RA Leiden, The Netherlands
2. Department of Epidemiology & Biostatistics, Amsterdam Public Health Research Institute, Amsterdam University Medical Centers, PO Box 7057, 1007 MB Amsterdam, The Netherlands
3. Mathematical Institute, Leiden University, PO Box 9512, 2300 RA Leiden, The Netherlands
4. Department of Epidemiology & Biostatistics, Amsterdam Public Health Research Institute, Amsterdam University Medical Centers, PO Box 7057, 1007 MB Amsterdam, The Netherlands and MRC Biostatistics Unit, University of Cambridge, Cambridge CB2 0SR, UK