Effective number of parties

In political science, the effective number of parties is a diversity index introduced by Laakso and Taagepera (1979) which provides for an adjusted number of political parties in a country's party system, weighted by their relative size. The measure is especially useful when comparing party systems across countries.

The size of a party 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 lower than the actual number of parties. The effective number of parties is a frequent operationalization for political fragmentation.

There are several common alternatives for how 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.