1. Department of Statistics; University of Milano-Bicocca; via Bicocca degli Arcimboldi 8 20126 Milan Italy
2. Department of Clinical Epidemiology; Leiden University Medical Center; PO Box 9600 2300 RC Leiden The Netherlands
3. Department of Medical Statistics and Bioinformatics; Leiden University Medical Center; PO Box 9600 2300 RC Leiden The Netherlands