How we count counts: The empirical effects of using coalitional potential to measure the effective number of parties

Despite its conceptual centrality to research in comparative politics and the fact that a single measure-the Laakso-Taagepera index (Ns)-is widely employed in empirical research, the question of what is the best way to “count” parties is still an open one. Among other alleged shortcomings, Ns has been criticized for over-weighting small parties, especially in the case of a one-party majority. Using seat shares data from over 300 elections, I have-for the first time-calculated Ns as well as an alternative measure (NBz) which employs Banzhaf scores, rather than seat shares, as weights. The Banzhaf index is a voting power index which calculates a party's voting power as a function of its coalitional potential. Though the two measures are highly correlated, I identify three particular party constellations in which the differences between Ns and NBz are significant and systematic.