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

Abstract

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.