Berend W Schreurs1, Stephanie L van der Pas2. 1. Department of Orthopaedics, Radboud University Medical Center, Nijmegen, 6500HB, Netherlands. Electronic address: wim.schreurs@radboudumc.nl. 2. Department of Mathematics, Leiden University, Leiden, Netherlands; Department of Medical Statistics and Bioinformatics, Leiden University Medical Center, Leiden, Netherlands.