K. Arzheimer, and E. Carter. Keele European Parties Research Unit, Working Paper 19 (http://www.keele.ac.uk/depts/spire/Working%20Papers/KEPRU/KEPRU%20Paper%2019.PDF), Keele, (2003)
K. Armingeon, P. Leimgruber, M. Beyeler, and S. Menegale. Institute of Political Science, University of Berne, http://www.ipw.unibe.ch/mitarbeiter/ru_armingeon/CPD_Set_en.asp (29.12.04), (2004)