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Effective number of parties
Just like Douglas Rae said
The effective number of parties (Eff N) was devised in the late 1970s by Markku Laakso and Rein Taagepera as a measure of party system fragmentation. It is based on a fractionalisation measure proposed in 1968 by Douglas W Rae.
Measurement and meaning
It can be measured at the level of votes (effective number of elective parties, Nv) or of seats (effective number of legislative parties, Ns). Comparing the two tells us about the degree of defractionalisation brought about by the conversion of votes into seats, which is central to the study of electoral systems. It generates a number that conveys information about fragmentation; for example, a figure of 4.14 tells us that the party system is as fragmented as if there were 4.14 equal-sized parties. The measure was outlined in: Markku Laakso and Rein Taagepera, ' "Effective" number of parties: a measure with application to West Europe', Comparative Political Studies 12:1 (1979), pp. 3–27.
More information
More information about the effective number of parties and about the least squares index (LSq, a measure of disproportionality) can be found on the accompanying site, which contains
(i) the values of the measures, at both elective and legislative level, for over 900 elections in more than 90 countries;
(ii) a downloadable file explaining in detail how to calculate the value of the measures for any given election outcome;
(iii) a downloadable file enabling users to calculate the value of the measures themselves for any election outcome.
There is also much more information in the book (now available in paperback) The Politics of Electoral Systems (Oxford University Press, 2008).
Another concept widely used in the analysis of electoral systems is the effective threshold.
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Last updated 14 May, 2008 3:09 PM