Measuring bias: Moving from two-party to three-party elections

One method for assessing the extent of electoral bias is that first developed by Brookes. This method decomposes bias into different elements, including efficiency of vote distribution as well as effects separately produced by electorate size and turnout. Brookes' method is used to measure electoral bias largely in two-party systems but the rise of third parties, particularly in recent UK elections, has prompted the search for a reliable alternative. This paper reports upon findings from an on-going research programme. The nature and theoretical underpinnings of different procedures that might be used for decomposing bias in the three-party case are outlined. Two main procedures are constructed and then tested against the results from actual elections. The evidence shows that these procedures produce similar findings in respect of the 2005 general election but differences emerge when earlier elections are considered. Research continues to assess whether these differences follow from the nature of party competition at each election or the particular procedure employed.