1. Faculty of Statistics, TU Dortmund University, 44227 Dortmund, Germany;
2. Mathematical Institute, Heinrich Heine University, 40225 Düsseldorf, Germany
3. Epidemiology Unit, German Rheumatism Research Centre, 10117 Berlin, Germany
4. Oslo Centre for Biostatistics and Epidemiology, Department of Biostatistics, University of Oslo, 0317 Oslo, Norway