The Challenge of Measuring Political Ideology

by John Sides on May 4, 2011 · 9 comments

in Institutions

There has been a recent exchange of views about how liberal Barack Obama really is—involving Ezra Klein and Nate Silver, among others.  This gets at the challenge of how we measure political ideology and whether we can make easy comparisons between Obama, previous presidents, other politicians then and now, and so on.  I asked Michael Bailey, political scientist at Georgetown, for his take.  For more on his work on this subject, see here.  Here is Mike:

Ezra Klein recently argued that President Obama is ideologically a 1990s Republican.   Nate Silver wasn’t convinced and argued Obama is simply a 2010s Democrat.  In part Silver relied on Poole and Rosenthal’s DW-NOMINATE scores which show Obama to the left of Johnson and Kennedy and to the right of Carter and Clinton.  Even as he uses these scores, Silver remains skeptical, arguing:

I’m not entirely persuaded that they can capture all of these dynamics. The system is essentially blind to the content of legislation, so if there are changes in the types of bills that Congress votes upon, there could be long-run ideological changes that are not well accounted for by the system. Measuring ideological change is one of the trickiest questions that political scientists face, and a complete treatment would require a thesis- or book-length approach.

Political scientists have long grappled with these issues.  Sometimes it is easy to reify the NOMINATE scores into actual ideology, but even a few examples make it clear this is a terrible idea.
For example, in the 1960s we know politicians moved left.  At the start of the decade, politicians debated fairly mild civil rights reforms in the face of Jim Crow laws; by the end of the decade, politicians debated how aggressively students should be bused to promote desegregation.  But if you look at the NOMINATE scores (in this the “Common Space” version of the scores which are designed to be more comparable across time than the DW-NOMINATE scores) you see nothing of this.  You see something like this figure: racists from the 1960s like Senators Eastland and Ellender are indistinguishable from the modern moderate Democrats like Fritz Hollings.  Eastland openly argued for white superiority on the Senate floor; Hollings voted to override Bush’es veto of the 1991 Civil Rights Act.  There is huge movement but you see none of it in NOMINATE scores:

NOMINATE scores do a reasonable job dividing up members of Congress by ideology within any given Congress, but if we want to answer the kind of question that Klein and Silver are posing, they don’t do the trick.
Hence, the intuition of Silver is correct: if we really want to talk about ideological change, we need to keep track of policies and where politicians move on these policies.  Klein is starting to do that and Silver had some good pushback; hopefully political scientists will be able to put those kind of intuitions into a formal analysis soon.

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