There is a concept in the philosophy of language called “Empty Name.” Basically, the idea is that that something has a name does not mean it actually exists. One might say that that something can be measured (indirectly, especially) does not mean that whatever is being measured actually exists.
DWNominate and its abuses (as far as I’m concerned) is something that always drives up the wall, and Kevin Drum on Mother Jones blog, has put on another example.
The argument is that Sanders would have been beaten in 2016 because he is, supposedly, very very liberal. That Sanders may well have been beaten, one cannot argue with too much since he did not run in the general election and all we have are counterfactuals. But whether he stood to lose because he was allegedly too liberal requires that, first, there is such a thing as “ideology” exists that can be clearly identified and measured and, second, that we have a measurement thereof that is reasonably reliable. Naturally, like everyone else who does not know (or, even many who do) how exactly DWNominate is calculated, he appeals to Sanders’ DWNominate scores to supposedly show that Sanders is very very liberal.
The problem, of course, is that DWNominate measures ideology only to the degree that legislators’ votes are products mostly of their ideology and all other considerations (e.g. “politics”) are just random noise. This is problematic for all manner of reasons, and gets worse when all manner of indirect evidence are mobilized to put political actors outside Congress (e.g. governors) on the same scale. Basically, someone is identified as a liberal in DWNominate scores if they vote with the Democrats and conservative if they vote with the Republicans. Someone is identified as very liberal if they vote against both the average Democrat and the average Republican often, but vote with the Democrats more than they do with Republican, and vice versa for the very conservative. Basically, Sanders’ DWNominate score shows that he’s an outsider who does not often vote with either Democratic or Republican establishments who nevertheless tends to vote with the Democrats, if he does side with one versus the other. In other words, his scores say nothing that we did not already know, except for the superficial and misleading appearance of precision due to its numerical nature.
This is not quite the same thing as saying that DWNominate has nothing to do with “ideology,” but the linkage is a lot more tenuous than what a simple number seems to indicate. The overall patterns are not nonsensical, but the specific numbers should not be given the due credence because they are nowhere near as precise as they seem to be. Yes, Sanders is probably more “liberal” than the average Democrat–but it is not clear what exactly that means, and his voting record, as captured in his DWNominate scores, does nothing to add any more clarity. Like “empty names” in the philosophy of language, DWNominate (and other quantifications of vague and poorly defined concepts, like ideology) do not refer to something that is terribly well defined. This is a danger in the drive to quantify everything: so we have numbers, but what do they really mean? If you don’t know the answer (and, speaking for myself, I could not explain “liberal” and “conservative” when I was in graduate school because I did not know what they really are, and I know even less what they really mean nowadays.), that could mean that the concepts you are trying to explain may not actually “really” exist, at least not in a form as obvious as one might think at first.