Effective number of parties

Some political scientists use the effective number of parties as a diversity index. The measure as introduced by Laakso and Rein Taagepera (1979), 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.

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.

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

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