James Joyner isn’t quite convinced of Jeff Jenkins’ argument that John Kerry is more conservative relative to Democratic presidents (historically) than George Bush is liberal, using Keith Poole and Howard Rosenthal’s NOMINATE method. James writes:
The problem I have with Poole’s coding methodology is that it’s excessively time bound. To compare Bush 43 to Reagan or Kerry to Carter ignores massive shifts in public opinion during those time periods. The “center” is not a spot on a map; it’s a median of current attitudes.
There are actually two versions of Poole and Rosenthal’s methodology. The version Jenkins apparently used for his analysis (from the description in the article) is called W-NOMINATE, and only looks at a particular Congressional session (e.g. the 107th Congress, from 2001 to 2003). There’s a second version, called DW-NOMINATE, that allows comparisons over time between Congresses. In other words, using W-NOMINATE is inappropriate for comparisons over time.* James goes on to write:
I’d think the ACU/ADA ratings are much more useful than Poole’s, since the comparison is made against one’s contemporaries.
Actually, ACU and ADA ratings are essentially interchangeable with W-NOMINATE first dimension scores. But I think James is critiquing Jenkins for something that Jenkins actually didn’t do (even though the article might lead you to think he did).
It seems to me there are two related questions here: is Bush more extreme than Kerry? and, are Bush and Kerry more extreme relative to their partisan predecessors? The first question was pretty clearly answered by Jenkins in the article. The second can’t be answered by the W-NOMINATE method that Jenkins used—which, given his indication that he deliberately simplified the analysis (by using W-NOMINATE instead of DW-NOMINATE), makes it seem odd that he tried to make comparisons over time. The question I think Jenkins answered is “are Bush and Kerry more extreme relative to predecessor presidents vis à vis the Congresses they faced”—and, for that comparison, W-NOMINATE or ADA/ACU scores would work equally as well.
Update: Jeff Jenkins has a comment at Dan’s place that clarifies the situation; he did use DW-NOMINATE for the interyear comparisons, but that point was lost in the editing process. So ignore the above paragraph. ☺ He has some interesting points too in regard to Poole and Rosenthal’s book, Congress: A Political-Economic History of Roll Call Voting.
Also worth pointing out is the forthcoming APSR piece by Doug Rivers, Josh Clinton, and Simon Jackman, “The Statistical Analysis of Roll-Call Data”. There's also a recent issue of Political Analysis in which all of the articles were on ideal-point estimation (which is the technical term for NOMINATE and the Rivers-Clinton-Jackman approach). And, if you want to do it yourself, Andrew Martin and Kevin Quinn have included the Rivers-Clinton-Jackman procedure in their MCMCpack package for GNU R.
* The zero point on the W-NOMINATE scale changes between Congresses, as does the unscaled variance, because there is no external anchor point. The W-NOMINATE procedure inherently normalizes the ideology scores on both dimensions to a normal distribution with mean zero and variance such that no score is outside (-1, 1).