Measuring party system institutionalization in democracies

Party system institutionalization (PSI) is regarded as a critical underpinning of democracy. However, the systematic study of PSI in democracies is constrained by weaknesses in existing measures, which are limited in coverage or comprehensiveness, and do not account for the latent nature of the concept, measurement error, and non-random missing data. This article presents a novel measure of PSI that uses a Bayesian latent variable measurement strategy to overcome extant measurement issues. The subsequent measure not only offers unmatched coverage and has demonstrated validity, but also exhibits more robust empirical associations with a range of outcomes related to the performance of democracy than existing measures. The measure should facilitate more integrated research on the causes and consequences of PSI in democracies around the world.