Peter Winker is professor of statistics and econometrics at the University Gießen. Natalja Menold is a senior researcher at the Center for Survey Design & Methodology at the GESIS – Leibniz Institute for the Social Sciences in Mannheim. Rolf Porst was a senior researcher at the Center for Survey Design & Methodology at the GESIS – Leibniz Institute for the Social Sciences in Mannheim until he retired in 2012.