Effective number of parties
The effective number of parties is a concept introduced by Laakso and Taagepera 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 or seat share in the parliament. 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 fragmentation of a party system.
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, 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 the square of each party’s proportion of all votes or seats. The proportions need to be normalised such that, for example, 50 per cent is 0.5 and 1 per cent is 0.01. This is also the formula for the inverse Simpson index, or the true diversity of order 2.
An alternative formula proposed by Golosov is
which is equivalent - if we only consider parties with at least one vote/seat - to
Here, n is the number of parties, the square of each party’s proportion of all votes or seats, and is the square of the largest party’s proportion of all votes or seats.
Values
The following table illustrates the difference between the values produced by the two formulas for eight hypothetical vote or seat constellations:| Constellation | Largest component, fractional share | Other components, fractional shares | N, Laakso-Taagepera | N, Golosov |
| A | 0.75 | 0.25 | 1.60 | 1.33 |
| B | 0.75 | 0.1, 15 at 0.01 | 1.74 | 1.42 |
| C | 0.55 | 0.45 | 1.98 | 1.82 |
| D | 0.55 | 3 at 0.1, 15 at 0.01 | 2.99 | 2.24 |
| E | 0.35 | 0.35, 0.3 | 2.99 | 2.90 |
| F | 0.35 | 5 at 0.1, 15 at 0.01 | 5.75 | 4.49 |
| G | 0.15 | 5 at 0.15, 0.1 | 6.90 | 6.89 |
| H | 0.15 | 7 at 0.1, 15 at 0.01 | 10.64 | 11.85 |