Effective Number of Parties

The effective number of parties is the standard measure of party-system fragmentation, introduced by Markku Laakso and Rein Taagepera in 1979. Rather than simply counting how many parties exist, it weights each party by its relative size, so that a handful of dominant parties count for more than a long tail of negligible ones. Formally it is the reciprocal of the Herfindahl concentration of party shares: N equals one divided by the sum of squared shares. Computed on vote shares it yields the effective number of electoral parties (ENEP); computed on seat shares it yields the effective number of parliamentary parties (ENPP). The index gives a single, intuitive number — roughly the count of equally sized parties that would produce the observed concentration — and is the workhorse for describing and comparing party systems.