This is a special issue of Studies. Originally the brainchild of former editor Keith Whittington of Princeton University, it is a symposium of articles that explore the value of NOMINATE scores for the study of American political development (APD). Our hope is that the symposium will launch a valuable conversation among our readers about whether and how to incorporate NOMINATE into their research practice.Footnote ¹

NOMINATE is a procedure developed by Keith T. Poole and Howard Rosenthal to facilitate the study of congressional politics. This procedure builds on premises derived from the spatial model of voting, and it uses information on roll call votes to calculate a set of numerical values that quantify the policy preferences of members of Congress. These numerical values are called NOMINATE scores, and they have been calculated for each member of Congress for every session of Congress that has ever met. The scores are taken by many researchers as valid estimates of a legislator's “ideal point,” or their most preferred location on a continuum of policy alternatives. Similarly, many researchers accept such scores as valid measures of legislator ideology.

For some time now, NOMINATE scores of different types have enjoyed broad and varied use in empirical research on legislative politics, though there has been a lively discussion about their proper interpretation. Perhaps the most commonly used are DW-NOMINATE scores, which are set up to permit the comparison of scores over time within each chamber. NOMINATE scores of all types have been slower to find a place in APD, however. As a result, many students of APD are not entirely familiar with the approach.

What is the theoretical framework underlying NOMINATE? How are NOMINATE scores estimated? How much value—and what sort of value—do they have for researchers interested in studying American politics as it has unfolded over time? How well does NOMINATE fit with the kind of scholarly approaches and intellectual sensibilities that are characteristic of APD? How capable is it of illuminating the types of puzzles that APD scholars are prone to pondering and answering the types of questions that we usually ask?

These are the questions that motivated our symposium, which is composed of six articles. It may be most profitable to read the articles in order, but they can be read out of order or on their own as well.

The first four articles make up the core of the symposium. In the first article, Phil Everson, Rick Valelly, Arjun Vishwanath, and Jim Wiseman provide an accessible introduction to NOMINATE for readers of Studies, sketching out its theoretical and conceptual underpinnings, laying out the basic intuitions behind the estimation method with a useful degree of technical detail, illustrating how NOMINATE might be applied to problems in APD, and suggesting how APD scholars might use NOMINATE to explore the sources of political polarization. The topic of polarization is taken up by Frances Lee in the next article, which explores whether the high degree of polarization as indicated by NOMINATE scores during the Gilded Age reflects wide ideological divergence between the two parties over the proper role of the federal government or if instead these scores track party conflict over patronage and distributive logrolls. In the third article, Devin Caughey and Eric Schickler ask how well NOMINATE scores capture ideological divisions in the Senate during the 1920s and how well they measure the sharp ideological change of Southern senators during the 1930s and 1940s, and they reflect on the circumstances when alternative approaches might be used as a supplement or substitute for NOMINATE scores. David A. Bateman and John Lapinski ask in the fourth article whether NOMINATE is suitable for meeting certain specific research needs of historically minded scholars, and they present methodological alternatives that enable researchers to model how the preferences of members change from one session of Congress to another, how the preferences of members vary across different issue areas, and how the preferences of members shift over the course of a single legislative session.

The last two articles offer extended reactions to the main articles.Footnote ² Nolan McCarty provides a detailed response to the main articles, making the case that NOMINATE scores measure something that bears a strong family resemblance to ideology; that “second dimension” NOMINATE scores cannot be readily interpreted as a “racial” dimension; and that the limitations of NOMINATE identified by the authors of the main articles can often be satisfactorily addressed with the application of less time and energy than implementing their proposed alternatives would require. Reading the articles in a very different way, Richard Bensel offers an epistemological reckoning with NOMINATE and cautions against its use by students of APD, arguing that it is fundamentally not possible, for fundamental theoretical reasons, to interpret the scores.

As readers of the symposium articles will come to understand, there are a range of views on NOMINATE and APD. This symposium surely cannot settle the many questions that it raises. But our hope that is that it will launch a broader conversation that will ultimately enhance the study of APD.