Two-partyism in electoral studies: empirical indicators

Due to their practical and theoretical significance, two-party configurations are important in electoral studies. However, in empirical cross-national analysis, the measurement of two-partyism is often substituted with measures of party system concentration or fragmentation, leading to undesirable information losses. This study overviews the existing measures of two-partyism that had been previously proposed, albeit rarely used, in political science, and introduces a new indicator that is transparent in its mathematical formula and fulfills theoretical requirements for measuring two-partyism. The properties of both existing and new indices are tested on a hypothetical dataset encompassing party configurations with widely varying structural characteristics, as well as on a real dataset covering elections held in 75 countries worldwide in 2001–2023. The analysis demonstrates the utility of the new index in comparative research while confirming the validity of the previously developed measures. The new index satisfies all criteria that can be applied to measures of bipartisanship, both theoretically and from the perspective of research practice. Therefore, it can serve as a useful addition to the toolkit of political research. Importantly, the data completeness requirements of the new measurement method are absolutely minimal. This allows its use for the analysis of historical electoral statistics, where all available election results are reduced to the shares of votes or seats obtained by the two largest parties.

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