A measure of party-system fragmentation that weights parties by size: one divided by the sum of the squared vote or seat shares of all parties.
In more detail
Devised by Markku Laakso and Rein Taagepera (1979). Two equal parties give a value of 2.0; one party with 70 per cent and another with 30 per cent gives about 1.7. Measures based on votes and on seats differ, and the gap shows how disproportional the electoral system is. It is a core indicator in Lijphart's executives–parties dimension and in studies of how electoral rules shape party systems. Its limitation is that it can mask a dominant party among many small ones.