Ornstein, Norman J., Thomas E. Mann, and Michael J. Malbin. 2002. Vital Statistics on Congress 2001-2002. Washington D.C.: The AEI Press.

Note: Data shows percentage of members voting with a majority of their party on party unity votes. Party unity votes are those roll calls on which a majority of Democrats vote against a majority of Republicans. Percentages are calculated to eliminate the impact of absences as follows: (unity)/(unity+opposition).