Edited by Francisco Cabrillo and Miguel A. Puchades-Navarro
Chapter 8: A general measure of the ‘effective’ number of parties in a political system
An important aspect of the analysis of electoral outcomes is the evaluation of the distribution of votes and/or seats across political parties. For example, assessing the proportionality of an electoral system requires a comparison of the distribution of seats with the distribution of votes (Gallagher, 1991; Lijphart, 1995). Another example is provided by measures of the ‘effective’ number of parties in a system. As Lijphart (1995) notes, the problem here is how to count parties of different size and the assumption in the comparative politics literature is that some kind of weighting is required to solve this problem. In essence, calculating the effective number of parties in a system involves assigning a scalar value to a vector of inter-party distribution of seats or votes. At one extreme, this scalar value will (should) equal the number of parties that actually contest the election. This will occur when votes (or seats) are distributed equally among the parties. At the other extreme, this scalar value will (should) equal unity. This will be the case when all votes (or seats) accrue to just one party. In between these extremes, the effective number of parties will be greater than one, but less than the number of parties contesting.
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