Party discipline and measures of partisanship

Vote-based measures of partisanship have had a huge impact on the recieved wisdom about parties in the U.S. Congress. This article employs a cutpoint model to analyze how five such measures respond to changes in preference di stributions and to different forms of behavior: party-based discipline and nonpartisan or undisciplined behavior. Three sets of findings are central t o ongoing research about parties in legislatures (1) The well-known and wid ely used party-voting score cannot discriminate between polar types of beha vior. (2) All five measures encourage erroneous inferences of party discipl ine when only intraparty preference homogeneity may be present. (3) Of the tour measures that can discriminate between partisan and nonpartisan behavi or, historic congressional averages are often nominally high on a 0-100 sca le, however, the averages tend to be closer to no-discipline expectations t han to party-discipline expectations. Cumulatively, these findings suggest that, labels notwithstanding, vote-based measures of partisanship are ineff ective instruments for detecting genuine party-based voting, party strength and leadership support.