Effective number of parties - Wikiwand (2025)

Some political scientists use the effective number of parties as a diversity index. The measure as introduced by Laakso and Rein Taagepera (1979),^[1] produces an adjusted number of political parties in a country's party system, weighted by their relative size. The measure is useful when comparing party systems across countries.^[2]

A jurisdiction's party system can be measured by either:

1. The effective number of electoral parties (ENEP) weights parties by their share of the vote.
2. The effective number of parliamentary parties (ENPP) weights parties by their share of seats in the legislature.

The number of parties equals the effective number of parties only when all parties have equal strength. In any other case, the effective number of parties is smaller than the number of actual parties. The effective number of parties is a frequent operationalization for political fragmentation. Conversely, political concentration can be seen by the share of power held large political parties.^[3]

Several common alternative methods are used to define the effective number of parties.^[4] John K. Wildgen's index of "hyperfractionalization" accords special weight to small parties.^[5] Juan Molinar's index gives special weight to the largest party.^[6] Dunleavy and Boucek provide a useful critique of the Molinar index.^[7]

The following table illustrates the difference between the values produced by the two formulas for eight hypothetical vote or seat constellations:

The effective number of parties can be predicted with the seat product model^[10][11] as , where M is the district magnitude and S is the assembly size.

For individual countries the values of effective number of number of parliamentary parties (ENPP) for the last available election is shown.^[12] Some of the highest effective number of parties are in Brazil, Belgium, and Bosnia and Herzegovina. European Parliament has an even higher effective number of parties if national parties are considered, yet a much lower effective number of parties if political groups of the European Parliament are considered.

• Duverger's law– Winner-takes-all voting systems tend to result in only two viable parties
• First-past-the-post voting– Plurality voting system
• Majoritarian representation– System favoring larger parties over smaller ones
• Multi-party system– Political system in which three or more parties can run for, and win in, national elections
• One-party state– State where one party may form a government
• Proportional representation– Voting system that makes outcomes proportional to vote totals
• Two-party system– Government system dominated by only two major political parties
• Vote splitting– Losing candidate affecting election result
• Electoral competition– Competition between political parties or candidates
• Micromega rule– Preferences of political parties for electoral systems