RELATED STORIES: Obama: Most Liberal Senator In 2007 • Key Vote Breakdown • How The Vote Ratings Are Calculated • PAST SENATE RATINGS: (Subscribers Only) 2006 · 2005 · 2004 · 2003 · 2002 · 2001 · 2000 · 1999 · 1998 · 1997 · 1996 · 1995 • PAST HOUSE RATINGS: (Subscribers Only) 2006 · 2005 · 2004 · 2003 · 2002 · 2001 · 2000 · 1999 · 1998 · 1997 · 1996 · 1995

National Journal's vote ratings are sure to generate attention this year because of Sen. Barack Obama's rating as the most liberal senator in 2007 and Sen. Hillary Rodham Clinton's rating as the 16th-most-liberal senator. What follows is an explanation of the vote ratings, written by National Journal Editor Charles Green.

National Journal is a weekly magazine that covers public policy and politics. It is not aligned with a political party or ideology.

Green: To see how House and Senate members compare with each other on an ideological scale.

Q: How do you pick the votes?

Green: Toward the end of every year, several National Journal reporters and editors separately sift through all of the year's roll-call votes to identify ones that might be appropriate for the vote ratings. The reporters and editors then meet and make the final selections.

Q: How do you determine which votes are "appropriate"?

Green: First we try to identify the most important House and Senate votes of the year. Then we look for votes that show ideological distinctions between members, even if the votes aren't necessarily pivotal. Finally, we try to make sure that a wide range of issue areas are represented, such as abortion, the budget, energy, environment, immigration, Iraq, national security, and taxation.

Q: Can you give an example of votes that show ideological distinctions?

Green: The Senate voted last year on whether to repeal the federal minimum wage. The outcome of the vote was never in doubt -- only 28 senators voted for the repeal; 69 voted against it. But the vote seemed to us to be worth including in the ratings because it showed the ideological differences between senators who thought that setting a minimum wage is an appropriate function of the federal government (what we termed the liberal position) and those who thought that such matters should be left to the states (what we termed the conservative position).

Q: Why don't you base the ratings on all of the roll-call votes, rather than just some of them?

Green: Last year there were 1,186 roll-call votes in the House and 442 in the Senate. Many of them are on relatively minor matters and are noncontroversial. Other votes fall along regional or other nonideological lines. We think that a rating based on key votes is more informative.

Q: When you selected the Senate votes for 2007, did you know that Sen. Obama was going to have the most liberal rating?

Green: No. In fact, we didn't even know whether he would qualify for a score. Under our system, a member of Congress gets a liberal and conservative score in each of three broad issue areas -- economic policy, social policy, and foreign policy. A member must participate in at least half of the votes in a category to get a score in that category. If a member gets a score in all three categories, he or she also gets a composite score, essentially an average of the three scores. If a member doesn't get a score in all three categories, he or she doesn't get a composite score. Obama and other presidential candidates were absent a fair amount in 2007, so we weren't sure if they would get composite scores. Obama's composite score is the basis for his label as the most liberal senator in 2007.

Q: When you selected the votes, were you keeping track of how Obama (or any other member of Congress) had voted?

Green: No.

Q: What happens after you select the votes?

Green: Polidata, a nonpartisan political data-analysis firm, downloaded lists of members' votes on our key votes from the House and Senate websites. We sent the lists to the Brookings Institution, which is under contract with National Journal to compute the vote ratings, based on a methodology that we devised.

Q: What does Brookings do?

Green: The Information Technology Services division of Brookings subjects each vote to something called a principal-components analysis. That's 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). 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. Members were then ranked from the most liberal to the most conservative in each issue area.

Q: Who came up with that process?

Green: The ratings system was first devised in 1981 under the direction of William Schneider, a political analyst and commentator, and a contributing editor to National Journal.

Q: Does Obama's rating mean that he's the most liberal senator?

Green: The rating is just for his votes in 2007. For his votes in 2006, he was ranked the 10th-most-liberal senator. For his votes in 2005, he was ranked the 16th-most-liberal senator.

Q: How often did Obama vote the liberal position in 2007?

Green: He participated in 66 of the 99 votes used for the ratings. He voted the liberal position 65 times.

Q: Aren't the labels "liberal" and "conservative" open to interpretation?

Green: Yes. On some matters, most people would agree on what constitutes a liberal position or a conservative position. On other matters, it's not as clear-cut. Some critics of the war in Iraq, for instance, argue that opposition to the war is a conservative position because it reflects a belief in limited government involvement in international affairs. But in National Journal's ratings, votes in opposition to the war are categorized as liberal. Labels such as "liberal" and "conservative" are just that -- labels. They are subject to debate. But as long as National Journal thinks there's a broad consensus about what these labels mean, we'll continue using them in our vote ratings.

Q: Why are you releasing the scores for Barack Obama and Hillary Rodham Clinton before you release the scores for all other members of Congress?

Green: Back in December, we decided that we would publish the ratings of the presidential candidates as soon as they became available, rather than wait until our annual Vote Ratings issue on March 8. We thought it would be irresponsible to keep those scores under wraps during the height of the presidential primary season.

Q: Can't you be accused of trying to influence the Super Tuesday election results by releasing the ratings now?

Green: The Super Tuesday timing is coincidental. We received the final vote ratings from Brookings on January 25. We decided to publish the Obama and Clinton scores in the next issue of National Journal. We spent the time between January 25 and January 31, when the magazine was sent to the printer, double-checking the ratings and preparing stories and tables about them.

Q: You keep referring to Obama and Clinton. What about John McCain?

Green: He didn't get a composite score for 2007 because he missed too many votes.

Q: Are you concerned that National Journal's 2007 rating of Obama as the most liberal senator will become an issue in the presidential campaign?

Green: We can't control how the vote ratings are used in the campaign. One reason for this Q&A is to try to anticipate possible questions and be as open as possible about how the ratings were determined.

Q: Didn't you go through the same situation four years ago?

Green: Yes. In 2004, National Journal rated Democratic presidential nominee John Kerry as the most liberal senator in 2003. The rating quickly became a talking point in the campaign, with President Bush, Vice President Cheney, and other Republicans using it to attack Kerry. For his part, Kerry called the rating a "laughable characterization." He said it was "absolutely the most ridiculous thing I've ever seen in my life."

Q: Have you made any changes in the vote rating system since then?

Green: We made one change. We decided that in order for a member of Congress to receive a composite rating, he or she needed to vote often enough to qualify for scores in each of the three issue categories-economic policy, social policy, and foreign policy-that we measure. In Kerry's case, he didn't vote often enough in 2003 to merit scores in the social-policy and foreign-affairs categories. His overall ranking was based on his score in the economic category.

Q: Why did you make the change?

Green: We didn't want to continue giving composite scores to members of Congress who missed most of the votes we selected.

Q: Why didn't you make the change before Kerry's rating was announced?

Green: The method we used to give Kerry a composite score was the method we had used in the past. To change the rules in the middle of the game, so to speak, after we learned Kerry's ranking, would have exposed us to charges of manipulating our rules for partisan reasons. We instituted the change the following year, before we knew the scores of any lawmakers.

Q: Do you think that the National Journal vote ratings are a valid way to judge a member of Congress?

Green: It's one way to assess a member of Congress, but by no means the only way. It's important to look at a member's effectiveness, character, judgment, and policy proposals, among other things. It's also valuable to look at vote ratings from other organizations -- from publications such as Congressional Quarterly and interest groups such as the League of Conservation Voters, the American Civil Liberties Union, and the American Conservative Union -- to get a rounded view.