C A C Prinsen1, C Apfelbacher2. 1. Department of Epidemiology and Biostatistics, VU University Medical Centre and the EMGO+ Institute for Health and Care Research, De Boelelaan 1089a, 1081 HV, Amsterdam, the Netherlands. c.prinsen@vumc.nl. 2. Medical Sociology, Department of Epidemiology and Preventive Medicine, University of Regensburg, Regensburg, Germany.