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The effective number of parties, N = 1/Σp(i)2 (where p(i) is the fractional share of the ith party), usually suffices to describe adequately a constellation of parties of different strengths. Difficulties arise when disparity in party sizes is such that the largest share (p1) surpasses 0.50 (meaning absolute dominance), while N still indicates a multi-party constellation. In such cases N(∞) = 1/p1 is proposed as a supplementary indicator: a value less than 2 indicates absolute dominance. An 'NP' index proposed earlier is a combination of N and N(∞); its values are close of those of N(∞), but NP sometimes falls below 2 even when many parties are relevant for coalition formation. Appendix A offers an alternative approach based on indices of deviation from a norm, but it proves cumbersome.

In the presence of a large lumped category of 'Other' parties the effective number of parties cannot be known exactly. Some approaches used produce large discrepancies. This note shows how the effective number still can be estimated with fair accuracy. The same issue arises with the measures of disproportionality between seats and votes. © 1997 Elsevier Science Ltd.

This note helps to explain how cabinet-level concentration of power is constrained by party level concentration of seats. Arend Lijphart's Patterns of Democracy (1999) measures concentration of executive power by the frequency of minimal winning and one-party cabinets (MW/OP), and party concentration by the effective number (N) of legislative parties. In his factor analysis, these highly correlated indices are the central features that distinguish consensual from majoritarian systems. The present study establishes a quantitative logical relationship leading from N to the major component of MW/OP, so as to explain the reasons behind Lijphart's empirical observation. The note analyses separately the frequency of various types of cabinet coalitions that Lijphart's book has lumped together. It also offers a new way to visualize the effective number of legislative parties, as twice the minimal number of parties needed to form a minimal winning coalition. © 2002, Sage Publications. All rights reserved.

This paper proposes a new method to evaluate the number of relevant parties in an assembly. The most widespread indicator of fragmentation used in comparative politics is the "Effective Number of Parties", designed in 1979 by M. Laakso and R.Taagepera. Taking both the number of parties and their relative weights into account, the ENP is arguably a good parsimonious operationalization of the number of "relevant"parties. This index however produces misleading results in single-party majority situations as it still indicates that more than one party is relevant in terms of government formation. We propose to modify the ENP formula by replacing proportions of seats by voting power measures. This improved index behaves more in line with Sartori's definition of relevance, without requiring additional information (such as policy positions) in its construction. We thus advocate for the use of our "Effective Number of Relevant Parties"in future comparative research.

This paper proposes a new method to evaluate the number of relevant parties in an assembly. The most widespread indicator of fragmentation used in comparative politics is the "Effective Number of Parties", designed in 1979 by M. Laakso and R.Taagepera. Taking both the number of parties and their relative weights into account, the ENP is arguably a good parsimonious operationalization of the number of "relevant"parties. This index however produces misleading results in single-party majority situations as it still indicates that more than one party is relevant in terms of government formation. We propose to modify the ENP formula by replacing proportions of seats by voting power measures. This improved index behaves more in line with Sartori's definition of relevance, without requiring additional information (such as policy positions) in its construction. We thus advocate for the use of our "Effective Number of Relevant Parties"in future comparative research.

Attempts to predict the number of political parties emerging in democracies have usually been based on one of two seemingly incompatible approaches: (1) the 'institutional' approach (e.g., Duverger's Law and Hypothesis) focuses on the nature of the electoral system and also on the number of seats per district; (2) the 'ideological' approach stresses the nature and extent of social cleavages. This article attempts a synthesis by showing that election system and cleavage type interact to affect the number of parties, with the former factor determined in part by the latter. Our most striking finding, however, is that the effective number of parties tends to be obtained by adding 'one' to the number of issue dimensions. Within this broader framework, Duverger's Law emerges as a special case for polities with a single issue dimension, and Duverger's Hypothesis is replaced by a much more quantitative prediction as to the effective number of parties. Copyright © 1985, Wiley Blackwell. All rights reserved

Despite its conceptual centrality to research in comparative politics and the fact that a single measure—the Laakso-Taagepera index (LT)—is nearly universally employed in empirical research, the question of what is the best way to "count" parties is still an open one. Among other alleged shortcomings, LT has been criticized for over-weighting small parties, especially in the case of a one-party majority. Using seat-shares data from over 300 elections, I have calculated LT as well as an alternative measure (BZ) which employs normalized Banzhaf scores rather than simple party seat shares, as weights. The Banzhaf index is a voting power index which calculates a party's voting power as a function of its coalitional potential. Though the two measures are highly correlated, I identify three particular party constellations in which the differences between LT and BZ are systematic and statistically significant. In all of these cases, and especially in the case of a one-party majority, I argue that BZ is a more accurate representation of the actual party system, after any given election, while LT is perhaps better interpreted as a measure of the shape of the party system more generally. These findings have many implications, including with respect to the categorization of party systems and the empirical validity of Duverger's Law.

Accurate evaluation of electoral systems requires precise measurement of both the disproportionality and the concentration of party systems. While the measurement of disproportionality has been investigated systematically and clear guidance for measurement exists, such treatment is lacking for the concentration dimension. This paper clarifies the theoretical concept of concentration in focusing on the directness of the voter-government link and differences between types of coalitions. In light of this concept, a new measure is introduced, and the different measures are compared theoretically as well as empirically. In conclusion, the effective number of parties measure provides a fruitful framework, but should be applied to the cabinet instead of the parliament level in order to clearly identify substantial features of party system concentration.

We illustrate the power of "logical models" (Taagepera, 2007) by offering a three-parameter model of the relationship between the effective number of parties and electoral turnout that makes use of the constraints on what parameter values are internally coherent given boundary conditions to specify functional form, and seeks not optimal curve fitting but rather a direct model testing. In our model, one parameter reflects an effect that generally acts to increase turnout as the effective number of parties increases, another an effect that generally acts to decrease turnout as the effective number of parties increases, while a third parameter allows for baseline variation in turnout across countries (or within countries across elections). We fit this model to district-level data from 237 elections held in 17 countries, representing a wide range of electoral system types generating multi-party contests, with over 20,000 district-election observations. The basic intuition, that turnout rises to a peak as the effective number of parties increases and then falls slowly, fits our data pretty well. © 2013 © 2013 Elections, Public Opinion & Parties.

We illustrate the power of "logical models" (Taagepera, 2007) by offering a three-parameter model of the relationship between the effective number of parties and electoral turnout that makes use of the constraints on what parameter values are internally coherent given boundary conditions to specify functional form, and seeks not optimal curve fitting but rather a direct model testing. In our model, one parameter reflects an effect that generally acts to increase turnout as the effective number of parties increases, another an effect that generally acts to decrease turnout as the effective number of parties increases, while a third parameter allows for baseline variation in turnout across countries (or within countries across elections). We fit this model to district-level data from 237 elections held in 17 countries, representing a wide range of electoral system types generating multi-party contests, with over 20,000 district-election observations. The basic intuition, that turnout rises to a peak as the effective number of parties increases and then falls slowly, fits our data pretty well. © 2013 © 2013 Elections, Public Opinion & Parties.

The mechanical effect of electoral systems, identified by Maurice Duverger, can be estimated by means of a quantitative model. The model predicts the range within which the effective number of parties in a district should fall for a given magnitude (number of seats) of the district. At the national level, a related model predicts the effective number of parties based on the effective magnitude and the number of seats in the national assembly. The institutional variables considered—magnitude and assembly size—define a great portion of the structural constraints within which a given country's politics must take place. The model developed provides a good fit to data in spite of its having been developed from outrageously simple starting assumptions. © 1993, American Political Science Association. All rights reserved.

В статье рассматривается процесс становления многопартийной системы на примере выборов в Государственную Думу Российской Федерации с 1993 по 2007 гг. Применение данных электоральной статистики и методики вычисления эффективного числа партий показывает, что для России в современных условиях наиболее приемлемой является двухпартийная система. ; In the article the process of formation of a multi-party system on an example of elections in the State Duma of the Russian Federation since 1993 till 2007 is considered. Application of the data of electoral statistics and technique of calculation of effective number of parties shows that the most comprehensible for Russia in modern conditions is the two-party system.

This paper proposes a model of a legislature, formed by several parties, which has to vote for or against a certain bill in the presence of a lobbyist interested in a certain vote outcome. We show that the ease with which the lobbyist can manipulate a legislature decision increases with the number of elected parties, and, consequently, decreases with an electoral threshold. On the other hand, a lower electoral threshold increases the representativeness of a legislature. We combine these two effects in a notion of fairness. We show the existence of an electoral threshold that optimizes the fairness of a political system, which is close to 1–5%. Namely, the optimal threshold (in our sense) is close to thresholds that exist in most parliamentary democracies.

Altres ajuts: COST Action IC1205 on Computational Social Choice ; This paper proposes a model of a legislature, formed by several parties, which has to vote for or against a certain bill in the presence of a lobbyist interested in a certain vote outcome. We show that the ease with which the lobbyist can manipulate a legislature decision increases with the number of elected parties, and, consequently, decreases with an electoral threshold. On the other hand, a lower electoral threshold increases the representativeness of a legislature. We combine these two effects in a notion of fairness. We show the existence of an electoral threshold that optimizes the fairness of a political system, which is close to 1-5%. Namely, the optimal threshold (in our sense) is close to thresholds that exist in most parliamentary democracies.

This paper proposes a model of a legislature, formed by several parties, which has to vote for or against a certain bill in the presence of a lobbyist interested in a certain vote outcome. We show that the ease with which the lobbyist can manipulate a legislature decision increases with the number of elected parties, and, consequently, decreases with an electoral threshold. On the other hand, a lower electoral threshold increases the representativeness of a legislature. We combine these two effects in a notion of fairness. We show the existence of an electoral threshold that optimizes the fairness of a political system, which is close to 1-5%. Namely, the optimal threshold (in our sense) is close to thresholds that exist in most parliamentary democracies.