Measuring judicial ideology

Theoretically informed and reliable ideology measures are necessary buildings blocks of an increasingly wide range of empirical models of judicial behavior. In this chapter reviews several prominent approaches to the measurement of judicial ideology. A particular attention is given to Item Response Theory (IRT) models. The application of IRT measurement models to judicial voting data has been one of the most important developments in the literature in the past 20 years. The IRT framework generates measures of ideology but also can offer additional insight into the nature of legal decision making. The challenges of estimating ideology across time and institutions are also discussed. After reviewing the major assumptions and tradeoffs of various measures of ideology, I conclude by highlighting several avenues for future applied research.