A complete information index for measuring the proportionality of electoral systems

A crucial property of an electoral system is the manner in which it translates a vote distribution into a pattern of representation. Measures of this property are important in assessing the relative merits of differing electoral systems. Galeotti proposes one useful index (Galeotti, G., 1994, On proportional non-representation, Public Choice, 80, 359-70). However, it is argued that this index is informationally incomplete. A complete information index is proposed and shown to perform better in an illustration using data from Sicilian provincial elections.