Effective number of parties

The effective number of parties is a concept introduced by Laakso and Taagepera (1979) which provides for an adjusted number of political parties in a country's party system. The idea behind this measure is to count parties and, at the same time, to weight the count by their relative strength. The relative strength refers to their vote share effective number of electoral parties (ENEP) or seat share in the parliament effective number of parliamentary parties (ENPP). This measure is especially useful when comparing party systems across countries, as is done in the field of political science. 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 lower than the actual number of parties. The effective number of parties is a frequent operationalization for the political fragmentation.

There are two major alternatives to the effective number of parties-measure. 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.

The measure is essentially equivalent to the Herfindahl–Hirschman index, a diversity index used in economics; the Simpson diversity index, which is a diversity index used in ecology; and the inverse participation ratio (IPR) in physics.