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 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. This measure is especially useful when comparing party systems across countries and is a frequent operationalization for the fragmentation of a party system.