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Michael H. Crespin, David W. Rohde, and Ryan J. Vander Wielen, "Measuring variations in party unity voting: An assessment of agenda effects," Party Politics, 19 (May 2013), 432-457. [Available at http://ppq.sagepub.com/content/vol19/issue3/ ]

First paragraph:
Measures of party divisiveness, commonly referred to as party unity scores, have been widely used for various purposes in the congressional literature and elsewhere. In fact, these party unity scores have even been a motivating factor in some scholarly dialogues (Brady et al., 1979; Collie and Brady, 1985; Cox and McCubbins, 1993; Rohde, 1991). At the same time, scholars also recognize the important influence the agenda has on shaping the roll-call record (Cox and Poole, 2002; Cox and McCubbins, 2005; Roberts, 2007). Specifically, certain types of votes are more inclined to generate conflict than others. Moreover, we know that there is considerable variation in the content of the agenda across time (Rhode, 1992).

Figures and Tables:
Table 1. Expected effect of vote-type on conventional party unity measure as share increases.
Figure 1. Percentage party unity by Congress including adjusted party unity and adjusted party unity excluding ultra-consensual votes.
Table 2. Vote-type by Congress.
Table 3. Party unity by vote-type.
Table 4. Percentage of roll-calls with majorities of at least 95 percent.
Figure 2. Variation in adjusted scores caused by index Congress selection.
Table 5. Bivariate regressions.
Table 6. Replication exercise.
Appendix A. Vote categorization rubric
Appendix B. Measurement values

Last Paragraph:
(First paragraph of conclusions) These results have both substantive and methodological implications for analyses using measures of aggregate partisanship (and more broadly for aggregate congressional voting measures generally). As a general statement, it is imperative that researchers understand the details of voting data before conducting empirical analyses of it. When studying voting longitudinally, for instance, one needs to be sure that the analysis is comparing 'apples to apples'. This holds for those examining party unity scores over a long time frame or other aggregate measures that are derived from roll-call votes. We have demonstrated that, without adequate controls the data do not meet this condition due to the sizeable variation in the roll-call agenda. We show that changes in the conventional measure of party unity are in no small way a reflection of changes in the agenda. Increases (decreases) in traditionally consensual vote-types can and do lead to decreases (increases) in party unity scores that are wholly independent of partisan disagreement or cohesion. Inferences based on many conventional measures may then be spurious. Instead, for the party unity score -- one such example of a party divisiveness measure -- we offer a modification that controls for the variation in the agenda and offers a more accurate comparison of party unity across Congresses. The method detailed above can easily be extended to other measures of aggregate party divisiveness/cohesion.

Last updated November 2013