1. Biostatistics, Institute for Molecular Life Sciences; Radboud university medical center; P.O. Box 9101 Nijmegen 6500 HB The Netherlands
2. Department of Medical Informatics, Biometry and Epidemiology; Ludwig-Maximilians University Munich; Marchioninistr. 15 Munich 81377 Germany
3. Department of Statistics; Ludwig-Maximilians University Munich; Ludwigstr. 33 Munich 80539 Germany
4. Department of Medical Statistics and Bioinformatics; Leiden University Medical Center; P.O. Box 9600 Leiden 2300 RC The Netherlands