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DW-NOMINATE Scores With Bootstrapped Standard Errors

Royce Carroll, Jeff Lewis, James Lo, Nolan McCarty, Keith Poole, and Howard Rosenthal

Updated 28 March 2008

This updated release of the DW-NOMINATE scores for the 1^st to the 110^th (1^st session) Congresses (1789 - 2007) contains parametric bootstrapped standard errors. For an explanation of the basic theory of the parametric bootstrap see:

Political Analysis

Jeffrey B. Lewis

Keith T. Poole

This research was made possible by NSF Grant 0611880 to Jeffrey B. Lewis, Keith T. Poole, and Howard Rosenthal. We will post the 1000 bootstrap trials and software for analyzing them within a few weeks. We will also post a detailed technical report on how the standard errors were computed. We thank the National Science Foundation and the San Diego Supercomputer Center for their support.

The files below contain DW-NOMINATE scores for the 1^st to the 110^th (1^st session) Congresses (1789 - 2007). For an explanation of how the data is organized and formatted read this text file.

As we (Poole and Rosenthal) explain in our book Ideology & Congress (the 2^nd edition of Congress: A Political-Economic History of Roll Call Voting), the first dimension can be interpreted in most periods as government intervention in the economy or liberal-conservative in the modern era. The 2^nd dimension picks up the conflict between North and South on Slavery before the Civil War and from the late 1930s through the mid-1970s, civil rights for African-Americans. After 1980 there is considerable evidence that the South realigns and the 2^nd dimension is no longer important. See our discussion of this period in our monograph: Income Redistribution and the Realignment of American Politics (joint with Nolan McCarty, 1997, AEI Press). Further discussion can also be found in Spatial Models of Parliamentary Voting by Keith Poole and in Polarized America (joint with Nolan McCarty).

In the House scaling, the second dimension weight is 0.3463 and the Beta parameter (proportional to 1/s where s is the standard deviation of the error) is 6.054. The correct classification was 87.04 percent with an APRE of 0.611 and a geometric mean probability of 0.749. The corresponding values for the Senate scaling are .4113 and 7.254, for the second dimension weight and Beta, respectively. The correct classification was 85.7 percent with an APRE of 0.578 and a geometric mean probability of 0.736.

In order to calculate distances from the DW-NOMINATE scores you must multiply the second dimension by the weight parameter. To calculate the choice probabilities you must apply both the second dimension weight and the Beta parameter. Use the Yea and Nay outcome coordinates with considerable caution because, as we explain in Ideology and Congress, they are poorly identified. However, the cutting line is identified and can be used safely.

If you have used previous releases of the DW-NOMINATE scores please go to the Current vs. Past Releases of DW-NOMINATE Scores page. The scores computed for the 1^st to the 110^th are essentially identical to the previous release (i.e., the corresponding scores for Congresses 1 to 109). However, when a new Congress is added to the dataset this will slightly change the scores for more recent members because their scores are estimated using their entire voting history. This will also slightly change the overall means of the dimensions. Finally, the past few Congresses are nearly unidimensional with correct classifications of 90 percent or better. Consequently, the overall fit of the DW-NOMINATE estimation has increased as recent Congresses have been added to the dataset. Also note that the 2^nd dimension weight can vary somewhat from scaling to scaling because the 2^nd dimension coordinates can be contracted/expanded slightly. The weight will compensate for this. The Current vs. Past Releases of DW-NOMINATE Scores page shows the corrections necessary to map the past scalings into the current scalings along with STATA files containing the current and past releases.

Please note that the House files now contain scores for most Presidents. For Presidents prior to Eisenhower these are based on roll calls corresponding to Presidential requests. These roll calls were compiled by an NSF project headed by Elaine Swift ( Study No. 3371, Database of Congressional Historical Statistics, 1789-1989). Many of these scores are based upon a small number of roll calls so use them with caution!

If you have questions or need help with these files please send us e-mail at jblewis@ucla.edu (Jeff Lewis) or KPoole@ucsd.edu (Keith Poole).

The format of the legislator files is:


 1.  Congress Number
 2.  ICPSR ID Number:  5 digit code assigned by the ICPSR as 
                       corrected by Howard Rosenthal and myself.
 3.  State Code:  2 digit ICPSR State Code.
 4.  Congressional District Number (0 if Senate)
 5.  State Name
 6.  Party Code:  100 = Dem., 200 = Repub. (See PARTY3.DAT)
 7.  Name
 8.  1st Dimension Coordinate
 9.  2nd Dimension Coordinate
10.  1st Dimension Bootstrapped Standard Error
11.  2nd Dimension Bootstrapped Standard Error
12.  Correlation Between 1st and 2nd Dimension Bootstrapped Estimates
13.  Log-Likelihood
14.  Number of Votes
15.  Number of Classification Errors
16.  Geometric Mean Probability

The format of the roll call files is:

 1.  Congress Number
 2   Roll Call Number
 3.  Spread on 1st Dimension    -- if the roll call was not scaled, there
 4.  Midpoint on 1st Dimension  -- are 0.000's in all four fields 
 5.  Spread on 2nd Dimension    --
 6   Midpoint on 2nd Dimension  --