In latent scaling applications, such as the positioning of political parties, differential item functioning (DIF) may occur because of measurement issues or because of substantive differences in the association between latent and manifest variables. While the first source of DIF has received considerable attention, the second has not, although it is of potential interest to comparative scholars. In this research note, we introduce a novel hierarchical Bayesian item response model that allows us to disentangle different sources of DIF. Drawing on the 2019 Chapel Hill Expert Survey (CHES), we highlight how the same issues are unequally politicized across Western Europe, and how some issues are less ideologically determined than others. Our model can be adapted to alternate settings, allowing researchers to shine a light on variation in, e.g., ideology, issue politicization, or party competition.

Scholars seeking to understand political competition in Europe have proposed various models of political dimensionality. While most scholars draw on data from the supply side of politics (political parties), demand-side (voter) studies remain few. In this article we compare the two approaches. The main difference is that while supply-side approaches suggest a single model of dimensionality that can be applied to all EU countries, demand-side approaches suggest a greater degree of divergence. In particular, the bundle of issues commonly identified by supply-side studies as TAN/GAL not only fail to form a coherent dimension when viewed from a demand-side perspective, but incorporate issues of EU integration in some (northern European) cases, but not in others.