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Bernard Grofman and Reuben Kline, "How many political parties are there, really? A new measure of the ideologically cognizable number of parties/party groupings," Party Politics 18 (July, 2012), 523-544. [Available at ]

First paragraph:
How many political parties are there in the parliament of country Q? How many parties contested the election in which that parliament was elected? To anyone who has not thought much about these questions, they seem rather trivial problems - requiring for their answer only the kind of counting you learned to do in kindergarten. Yet, they are far from simple once we recognize that, as social scientists, what we want to do is to operationalize the number of parties at the electoral and parliamentary level in a way that will allow us to forge theoretical links between these variables and other key features of a country's politics. After reviewing the two most important efforts to address such questions: the Laakso-Taagepera index (LT index) of the effective number of parties (Laakso and Taagepera, 1979) and the Banzhaf power score modification of the LT index to take into account party decisiveness in a parliamentary weighted voting game (Dumont and Caulier, 2003; Grofman, 2006; Kline, 2009), which we will abbreviate the LTB index, we offer a new method of counting parties that integrates effective size considerations with party locations in policy space to develop what may be thought of as a measure of the ideologically cognizable number of parties/ party ideological groupings.

Figures and Tables:
Figure 1. Hypothetical Party Locations in a Three Party System
Table 1. Dislocation for the Hypothetical Party Configuration in Figure 1(a)
Table 2. Canadian Paty System Data, 2004 Election
Table 3. Slovenian Paty System Data, 1996 Election

Last Paragraph:
With appropriate data, the methodology we have used can be applied to many more countries, just as is true for the Dalton (2008) polarization measure, and it need not be restricted to one-dimensional representations of party space. It is increasingly common in studies of party systems or electoral competition to report a time series for Laakso-Taagepera values at the vote and/or seat level. Our methodology can also be applied to making sense of changes in party constellations over time in away that is usefully complementary to the more standard approaches of simply counting changes in seat-winning parties or in the effective number of parties.Wehope that this articlewill inspire the development of similar time series on changes in the ideological bloc structure of party competition.

Last updated August 2012