Characterising Electoral Systems: An Empirical Application of Aggregated Threshold Functions

This article proposes a new way to measure proportionality using aggregated threshold functions. Electoral systems can be summarised by a single value that shows the necessary share of the total vote to win either one seat or half of the seats in parliament. The article calculates aggregate threshold values for 142 different electoral systems that were used in 525 democratic elections between 1946 and 2000. These results are also contrasted with the most commonly used indices of proportionality and turn out to be both substantively and empirically richer. Aggregated threshold functions provide both students and reformers of electoral systems with a measure based purely on institutional variables that offers an exhaustive summary of the functioning of many electoral systems.