Here are two more takes at it. First, courtesy of UNC Ph.D. student Brice Acree, takes my original plot and then adds an underlying measure of uncertainty—essentially, the margin of error for the estimated margin of victory.
Second, spurred on by a friend who is also a pollster, I calculated a different quantity—called “A” in the piece by Martin, Traugott, and Kennedy—that also captures predictive accuracy, is more robust to how polls treat undecided voters, and allows me to calculate a confidence interval. (UPDATE: This quantity was first calculated in the analysis by Costas Panagopoulos that I mentioned in my first post. However, I mistakenly overlooked this fact. Thus, what I present here is an unintentional replication of what he did.) Here, I included only polls that left the field on November 3 or later, in an effort to exclude polls who polled earlier and may simply have missed late movement in the polls (as opposed to being inaccurate per se). I averaged the two PPP polls during this period.
Although A has no intuitive metric, the results are sensible. Rasmussen and Gallup, for example, had a more pro-Romney bias. (“Bias” here is a statistical term of art and does not imply any partisan agenda on their part.) PPP, YouGov, and Ipsos/Reuters had almost no bias. Other polls, like ABC/Washington Post and NBC/WSJ had very minimal bias.
But note that because we are looking mostly at individual polls without very large samples, the underlying uncertainty is large. [UPDATE: Note that this is what Panagopoulos observed as well—“but none of the 28 national pre-election polls I examined had a significant partisan bias.”]