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 (N_s)-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, N_s 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 N_s as well as an alternative measure (N_Bz) 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 N_s and N_Bz are significant and systematic.