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Grigorii V. Golosov, "The Effective Number of Parties: A New Approach," Party Politics, 16 (March 2010), 171-192. [Available at http://ppq.sagepub.com/content/vol16/issue2/.]

First paragraph:
The 'effective' number of parties is a very simple general concept that entails a complex methodological problem. Conceptually, the effective number of parties is simply the number of 'viable' or 'important' or (to put it most radically) 'real' political parties in a party system that includes parties of unequal size. In comparative political party research, we need to distinguish between a four-party constellation with party vote-shares (0.52, 0.45, 0.02, 0.01), which is 'effectively' a two-party system, and a four-party constellation with party vote-shares (0.40, 0.25, 0.20, 0.15), which is 'effectively' a case of multipartism. There should be a way to discount very small parties. The methodological problem is that there are none readily available. Of course, it is possible to set an arbitrary threshold of exclusion, discounting all parties that fail to reach 1 or 3 or 5 percent of the vote.1 Yet it is clear that a 5 percent party might be unimportant if other parties enjoy massive support, and it might be quite important if other parties are comparably weak; for instance, if the largest party's vote-share is 15 percent. Then we need to quantify the idea of the 'effective' number of parties in a systematic way that allows for taking into account the relative sizes of parties, which is impossible without using a measure expressed as a mathematical formula.

Figures and Tables:
Table 1. Values of NLT, NB and NP for eight hypothetical vote or seat constellations
Table 2. Aggregate values of three effective number of parties indices for 38 seat distributions in sub-Saharan Africa and 42 vote distributions in East Central Europe/former Soviet Union
Figure 1. Distribution of 80 party constellations across the spaces of the Laakso-Taagepera (NLT) and new (NP) effective number of parties indices
Appendix 1. Cases and the Data for the Empirical Test

Last Paragraph:
While stating that there was no perfect measure of the effective number of parties became commonplace in scholarly papers dealing with the matter, and while the validity of such claims cannot be denied on philosophical grounds, a possible extension of this philosophy is that there is always room for improvement. If the effective number of parties, as defined by Laakso and Taagepera, tends to produce unrealistically high scores for very concentrated party systems, thus failing on intuitive content, why not develop a measure that is devoid of such a shortcoming? And, even though a partial solution to this problem is already available, in the form of the Dunleavy-Boucek index, why not eliminate the problem completely? This is what I have attempted to do in this study. The new index, defined as , solves several problems faced by those who need to count the effective number of parties. First, it satisfies all basic requirements of indices of this kind. Second, it produces reasonably small scores for party constellations that appear to have few important parties. Third, it registers many parties in those constellations where there seems to be many parties. Fourth, it minimizes undesirable side effects such as the 'kink' effect. This combination of properties makes the proposed index superior to the earlier ways of counting the effective number of parties.

Last updated March 2010