Description of NOMINATE Data

(File names and formats follow these Descriptions)

13 July 2004

There are 4 kinds of NOMINATE Coordinates:

  1. The original D-NOMINATE Coordinates

    These 2-dimensional dynamic coordinates were originally estimated on the CYBER 205 supercomputer at Purdue University during 1988-89 and they are extensively analyzed in our book Congress: A Political-Economic History of Roll Call Voting, Oxford University Press, 1997). Each legislator's point is dynamic and is allowed to move as a linear function of time as measured by the Congress number (higher polynomials in time did not appreciably increase the fit). A legislator's point is constant within a Congress but "jumps" along a linear path between Congresses. A member had to serve at least 3 Congresses for the linear model to be estimated.

    Because of the "overlapping generations" nature of the estimation, scores in one Congress are directly comparable with scores in another Congress. However, as we caution in our book, cross-Congress comparisons should be conducted only between Congresses occurring during one of the stable 2-party periods of American History. Also, the D-NOMINATE scores cannot be compared across chambers.

    Please see Chapters 1 -3 of our book for a description of the spatial model underlying D-NOMINATE and Appendix A of our book for the technical details and Monte-Carlo tests of the D-NOMINATE procedure.
  2. W-NOMINATE Coordinates

    W-NOMINATE is a static (i.e., meant to be applied to only one Congress) version of D-NOMINATE, with a number of improvements being designed to increase the efficiency of the algorithm so that it can be run on a desktop personal computer. To this end W-NOMINATE differs from D-NOMINATE in two ways: It uses a slightly different deterministic utility function; and, because it is a static algorithm, it constrains the legislators and roll call midpoints to lie within an s-dimensional hypersphere of radius one (in contrast to the rather flexible constraint structure necessitated by the dynamic model).

    The original D-NOMINATE deterministic utility function was:

    uijy = b e[- (w2dijy2)/2]

    which is simply a normal distribution multiplied by a constant. Note that there is no index on w -- namely the weight is the same for all dimensions. Experimentally, we found that estimating both b and w was unnecesary so we set w2/2 = 1/8 (see Appendix A of our book for all the details).

    The W-NOMINATE deterministic utility function is:

    uijy = b e[- åk=1,s (wk2dijy2)/2]

    The weight on the first dimension, w1, is set equal to one and the other dimension weights are estimated (see Appendix A of our book for all the details).

    The W-NOMINATE coordinates are highly correlated with the D-NOMINATE coordinates for most Congresses (Pearson r's typically greater than .95 for both the 1st and 2nd dimensions). However, unlike the D-NOMINATE scores, W-NOMINATE scores are not directly comparable between Congresses.

    See Appendix A of our book for the technical details of the W-NOMINATE procedure. Jeff Lewis and I are computing Legislator Coordinate files with parametric bootstrapped standard errors for all Houses and Senates in American History. They can be found on the Parametric Bootstrap Page. For an explanation of how these are computed see:
"Measuring Bias and Uncertainty in Ideal Point Estimates via the Parametric Bootstrap" by Jeffrey B. Lewis and Keith T. Poole.

File Names and Formats for NOMINATE Data

  1. KREH01.ORD to KREH99.ORD and KRES01.ORD to KRES99.ORD are D-NOMINATE coordinates. The format of the D-NOMINATE files is:
    
     1.  Congress Number
     2.  ICPSR ID Number:  5 digit code assigned by the ICPSR as 
                           corrected by Howard Rosenthal and myself.
     3.  ICPSR ID Number:  original 5 digit ICPSR code (this field is
                           usually blank).
     4.  State Code:  2 digit ICPSR State Code.
     5.  Congressional District Number (0 if Senate)
     6.  State Name
     7.  Party Code:  100 = Dem., 200 = Repub. (See PARTY3.DAT)
     8.  ICPSR Occupancy Code:  See any ICPSR Roll Call Voting Codebook
     9.  ICPSR Office Code:  See any ICPSR Roll Call Voting Codebook
    10.  Name
    11.  1st Dimension Coordinate
    12.  2nd Dimension Coordinate
    
    Note that if the 1st and 2nd dimension coordinates are exactly .000, then the Senator/Representative did not have enough roll call votes to be estimated in that Congress.

    To download these files go to the D-NOMINATE Scores Page.
  2. W-NOMINATE coordinates.

    
     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
     8-11. The cross-classications for the legislator.  The two middle numbers are the "errors".  
           For example, in the 106th House Callahan's cross classification numbers are:
            CALLAHAN     494  50  20 278  
            or in table form
                             ACTUAL
                           Yea     Nay
                     Yea   494      50
         PREDICTED
                     Nay    20     278
    12.  Geometric Mean Probability
    13.  1st Dimension Coordinate
    14.  2nd Dimension Coordinate
    15.  1st Dimension Standard Error
    16.  2nd Dimension Standard Error
    W-NOMINATE scores With Bootstrapped Standard Errors

    W-NOMINATE scores by year for 1939 - 1998

    W-NOMINATE Program
  3. BL75107.DAT are the Common Space coordinates -- W-NOMINATE coordinates for the 75th through the 107th Houses and Senates adjusted so that the the House and Senate members and the Presidents are in the same space. There is one record for each legislator in each Congress for this time period (n=17,871, 1937-2002). For an explanation of how this was done, see "Estimating a Basic Space From a Set of Issue Scales," American Journal of Political Science, 42 (July 1998), pp. 954-993.

    The format for BL75107.DAT is:
    
              1.  Congress Number 1 - 107
              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.  Congress District Number (Senate and President = 0)
              5.  Name of State
              6.  Party Code:  100 = Dem., 200 = Repub. (See PARTY3.DAT)
              7.  Name
              8.  1st Dimension Coordinate
              9.  2nd Dimension Coordinate
             10.  Indicator For Chamber
                                0 = House
                                1 = Senate
                                2 = President (83 - 107)
    
    To download these files go to the Common Space Page.
  4. H761939.DAT to H1051998.DAT and S861959.DAT to S1051998.DAT are yearly W-NOMINATE coordinates.

    The format of these files is the same as the D-NOMINATE and W-NOMINATE files.

    W-NOMINATE scores by year for 1939 - 1998
  5. HL01107A1.DAT and SL01107A1.DAT are DW-NOMINATE legislator coordinates for the 1st to the 107th Congresses. HC01107A1.DAT and SC01107A1.DAT are the corresponding roll call coordinates.
    
    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 Conditional Standard Error
    11.  2nd Dimension Conditional Standard Error
    12.  Log-Likelihood
    13.  Number of Votes
    14.  Number of Classification Errors
    15.  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  --
    
    To download these files go to the DW-NOMINATE Page.
Note that the sign of the 2nd dimension D-NOMINATE coordinates for the Senate is the reverse of that for the House. We multiplied the Senate 2nd dimension coordinates by -1.0 when we used them in our published work so that they would be comparable to the House coordinates.

The state, congressional district, and party codes correspond to Ken Martis's The Historical Atlas of Political Parties in the United States Congress. Howard and I consider Martis's Atlas to be the best source of information about party affiliation of members of Congress so that our data base can be considered to be very "clean". Congresses 1 - 100 have been extensively checked by ourselves and other scholars. Congresses 101-107 have also been checked but cannot be considered to be completely "clean". If you find errors, please send E-Mail to Keith Poole ktpoole@uga.edu so that corrections can be made.

The file PARTY3.DAT contains the dictionary for the party codes. Please cite Ken Martis's atlas as the original source if you use these codes in any way.



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