We show how to extend the Laakso-Taagepera measure of the effective number of parties so as to incorporate both party and ethnicity in a way that allows both ethnic-specific, party-specific, and composite measures that we refer to as extended L-T indices. While the aim of this article is methodological, we also illustrate our approach with U.S. two-party data from presidential elections, using two-group, three-group, and five-group categorizations of ethnicities to show how party ethnicization in that country has grown since 1952. For comparison, and to show the generality of our approach to situations with more than two parties, we present data from the Federation of Bosnia and Herzegovina (Federacija Bosne i Hercegovine), where we have three ethnic groups but also more than two parties.
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