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") or seat share in the parliament ("effective number of parliamentary parties"). This measure is especially useful when comparing electoral 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 fragmentation of a party system.
Contents
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.
Formulae
According to Laakso and Taagepera (1979) the effective number of parties is computed by the following formula:
Where n is the number of parties with at least one vote/seat and
An alternative formula proposed by Golosov (2010) is
which is equivalent - if we only consider parties with at least one vote/seat - to
Here, n is the number of parties,
Values
The following table illustrates the difference between the values produced by the two formulas for eight hypothetical vote or seat constellations: