A few weeks ago there was an election in Virginia where the polls were off by a fair amount—the margin of victory for the Democrat in the governor's race was about five points less than the polling average had predicted.
The question I wanted to answer is, just how unusual is it for such a polling miss to occur?
The simple answer: it's pretty much normal when there's a third candidate polling more than 5 percent.
We already know that a substantial portion of polling error is related to Obama's 2008 vote share in that state, so we need to take that into account when considering what the typical polling error would be.
Here's a graph of the error in the margin predicted by polling plotted against Obama's 2008 vote share, for elections between 2003-2013 with more than five polls in the polling average.
Clik here to view.
Learn more about what this graph means below the fold.