W. de Groen, and Z. Kilhoffer. Working Paper, WPEF19058. European Foundation for the Improvement of Living and Working Conditions, Dublin, (September 2019)
J. Axenbeck, A. Berner, and T. Kneib. ZEW Discussion Paper, 22-059. ZEW – Leibniz-Zentrum für Europäische Wirtschaftsforschung, Mannheim, (November 2022)