What the DW-NOMINATE ideology score measures
A description of voting patterns, not a rating of a legislator.
DW-NOMINATE is a statistical method developed by political scientists Keith Poole and Howard Rosenthal. It places each legislator on a scale using only their recorded votes.
What goes in
Roll call votes. Nothing else. Not speeches, not sponsorships, not press releases, not campaign finance. If a member never votes, they have no score.
What comes out
A number roughly between −1 and +1 on the first dimension. Members who vote together get similar scores. In the modern Congress the first dimension corresponds closely to the liberal–conservative divide, but that correspondence is an observed result, not a built-in assumption.
What it does not measure
Effectiveness. Attendance. Whether a member's votes matched their district. Whether any particular vote was correct. A score near zero means a member votes with both coalitions at similar rates — it does not mean they are moderate in their views, and it is not a compliment or a criticism.
Why we show it
Because it is a compact, reproducible summary of a voting record that anyone can check against the same public data. We label the source on every profile that carries one.