Mapping the Indices of Seats–Votes Disproportionality and Inter-Election Volatility

Abstract

Measures of electoral system disproportionality and of party system volatility (as well as malapportionment and vote splitting) present similar statistical issues in terms of deciding what index is most appropriate, but it is not common to view indices of disproportionality and volatility as serving similar ends. Making use of 12 different criteria, we evaluate 19 indices that have been previously proposed as measures of either disproportionality of electoral seats–votes results or over-time volatility of party vote (or seat) shares. We suggest that, on balance, Gallagher’s (1991) index, which has achieved increasing acceptance in the seats–votes literature on disproportionality (see esp. Lijphart, 1994) offers the most desirable combination of features, although the advantages it offers over the Loosemore-Hanby index are not large and are debatable. We also find that Dalton’s principle of transfers presents an ambiguity when one party has a larger number of excess seats, while another has a larger proportion of them.