Thanks Jeff for the comment to my first post and let me amplify in this second post. I agree that accurately measuring judicial ideology is crucial to so much of our research in judicial politics. To answer Jeff’s concerns, the scores are very highly correlated with the Giles et al. common space scores (pearson=.768), and the Giles’ scores do not predict lifetime liberalism of federal judges significantly better or worse than the Nixon/Howard scores (pearson correlation=-.278, p<.01). Having said that, however, there is a problem with the Giles et al measure in predicting judicial voting behavior. When there is no home state senator of the same party as the president, Giles et al. scores the nominee at the same ideology as the president. But because the point estimates for presidential common space scores are relatively extreme (see Clinton et al. 2004), this assumption has the effect of making these judges ideologically extreme. Indeed if both Indiana senators were Republican then Hamilton would have the same ideology score of Obama, which although not as liberal as conservative commentators make it out to be, it is considerably to the left of Bayh and our measure.
The assumption is a significant one, because senatorial courtesy was absent in about one of four of 395 appointments to the U.S. Circuit Courts of Appeals. Scott Graves and I had to confront this problem in our book on recess appointees and comparing pre and post recess appointment voting. A recess appointment by definition has no home state senator and the Giles et al. measures just did not make sense and that the voting behavior of judges does not support the extreme. Judges appointed by Republican presidents exhibit only a slightly more conservative voting record when their appointment was unconstrained by senatorial courtesy, and the difference is not statistically significant by a 1-tailed test. Judges appointed by Democratic presidents exhibit only a slightly more liberal voting record when their appointment was unconstrained by senatorial courtesy, and the difference is not statistically significant. In comparison, the Nixon/Howard ideology scores show that the pattern much more closely matches observed voting behavior of judges. The scores exhibit slight extremity in the president’s direction, when the president’s nomination is unconstrained by senatorial courtesy, and the difference is only marginally significant.
Of course one could argue that in the absence of a moderate Democratic senator like Bayh, Obama would have appointed a more liberal judge, but given the pool of available judges that does not seem likely and the above referenced data do not support such a conclusion