The measurement of political cleavages: a new index

The study of the relationship between social structure and political behavior has a long and notable history in political science and political sociology. One specialized component of structural analyses is the measurement of party cleavages based on social cleavages. A brief review of these measures shows they have developed from the relative simplicity of Alford’s Index through the increasing complexity of Thomsen, kappa and lambda indices. In the process, political cleavage measures have become more distant from the distribution of votes they are meant to summarize and, as a result, more difficult to articulate and interpret. In this paper, I propose a new party cleavage measure I call the delta index and illustrate it with data from the 2000–2020 American National Election Studies and the Pew Research Center’s 2014 Religious Landscape Study. Expressed as a percentage, the delta index retains the easy interpretability of the Alford Index while also incorporating important measurement advances introduced by kappa and lambda indices. In sum, the delta index provides a straightforward, accessible, and flexible methodological tool for quantitative political scientists and political sociologists, especially those seeking to speak to academic colleagues outside quantitative specialties and to broader audiences outside the academy.