Ends Against the Middle: Introducing the Generalized Graded Unfolding Model for Non-monotonic Item Response Functions

March 15, 2019 12:00PM E53-482

Standard methods for measuring ideology from voting records assume strict monotonicity of responses in individuals’ latent traits. If this assumption holds, we should not observe instances where individuals at the extremes act together in opposition to moderates. In practice, however, there are many times when individuals from both extremes may behave identically but for opposing reasons. For example, both liberal and conservative justices may dissent from the same Supreme Court decision but provide ideologically contradictory reasons. In this paper, we introduce to the political science literature the generalized graded unfolding model (GGUM), first proposed by Roberts, Donoghue, and Laughlin (2000), which accommodates non-monotonic response functions consistent with single-peaked preferences. In addition to explaining the method, we provide a novel estimation method and software that outperforms existing routines. We then apply this method to voting data from the U.S. Supreme Court and Congress and show that the GGUM outperforms standard methods in terms of both predictive accuracy and substantive insights.

Political Methodology Speaker Series