A panel of National Journal editors and reporters initially compiled a list of 196 key congressional roll-call votes for 2009 -- 99 votes for the Senate and 97 for the House -- and classified them as relating to economic, social, or foreign policy. The ratings system was first devised in 1981 under the direction of Bill Schneider, a political analyst and commentator, and a contributing editor to National Journal.
Lists were downloaded from the House and Senate websites showing how all the members voted on the key votes. The votes in each issue area were then subjected to a principal-components analysis, a statistical procedure designed to determine the degree to which each vote resembled other votes in the same category (the same members tending to vote together).
Five of the 196 votes (all in the House) were dropped from the analysis because they were statistically unrelated to others in the same issue area. These typically were votes that reflected regional and special-interest concerns, rather than general ideology.
The analysis also revealed which yea votes correlated with which nay votes within each issue area (members voting yea on certain issues tended to vote nay on others). The yea and nay positions on each roll call were then identified as conservative or liberal.
Each roll-call vote was assigned a weight from 1 (lowest) to 3 (highest), based on the degree to which it correlated with other votes in the same issue area. A higher weight means that a vote was more strongly correlated with other votes and was therefore a better test of economic, social, or foreign-policy ideology. The votes in each issue area were combined in an index (liberal or conservative votes as a percentage of total votes cast, with each vote weighted 1, 2, or 3).
Absences and abstentions were not counted; instead, the percentage base was adjusted to compensate for missed roll calls. A member who missed more than half of the votes in any issue category was scored as "missing" in that category (shown as an asterisk [*] in the vote-rating tables).
Members were then ranked from the most liberal to the most conservative in each issue area. These rankings were used to assign liberal and conservative percentile ratings to all members of Congress.
The liberal percentile score means that the member voted more liberal than that percentage of his or her colleagues in that issue area in 2009. The conservative figure means that the member voted more conservative than that percentage of his or her colleagues.
For example, a House member in the 30th percentile of liberals and the 60th percentile of conservatives on economic issues voted more liberal than 30 percent of the House and more conservative than 60 percent of the House on those issues, and was tied with the remaining 10 percent. The scores do not mean that the member voted liberal 30 percent of the time and voted conservative 60 percent of the time.
Percentile scores can range from a minimum of 0 to a maximum of 100. Some members, however, voted either consistently liberal or consistently conservative on every roll call. As a result, there are ties at both the liberal and the conservative ends of each scale. For that reason, the maximum percentiles are usually less than 100.
Members also receive a composite liberal score and a composite conservative score, each of which is an average of their six issue-based scores. Members who missed more than half of the votes in any of the three issue categories do not receive composite scores (shown as an asterisk [*] in the vote-rating tables).
To determine a member's composite liberal score, for example, first add the liberal scores in all three issue areas. Next, in each issue area, calculate 100 minus the member's conservative score and add the three results together. The two figures are then combined and divided by 6 (the number of individual scores).
This article appears in the February 27, 2010 edition of National Journal Magazine Contents.