A fully general index of political competition

As Selb [A deeper look at the proportionality-turnout nexus. Comparative Political Studies, forthcoming] observes, the standard measure of (posterior) competitiveness in elections used in plurality contests, namely the difference between the winner and the second place finisher, has no “self-evident counterpart in multi-member PR districts”. Following lines similar to Taagepera and Grofman [2003. Mapping the indices of seats–votes disproportionality and inter-election volatility. Party Politics 9, 659–677] we identify six properties that any index of competition should satisfy and then offer a new measure that satisfies all six properties which is applicable to virtually any electoral rule. We then apply our new and more fully general index of political competition to data from PR elections in Switzerland to see the extent to which the index of competition is correlated with levels of voter turnout.