The Decomposition of Electoral Bias in a Plurality Election

Abstract

This paper describes an empirical procedure for analysing the difference between the proportion of votes a major party attracts and the proportion of seats it subsequently wins in a plurality election. This difference will be referred to as the electoral bias with respect to that particular party at the election being analysed:B = S — V, (1)where S is the seat proportion and V the vote proportion gained by the party. This bias is the basic concept underlying much of the debate about electoral reform but curiously it has been an under-researched topic in political science literature. Apart from a small continuing debate over the cubic law of seat proportions and some analyses of Australian and New Zealand elections most attention in election studies has been concerned with determining the proportion of votes rather than the subsequent proportion of seats.