Polling in the 2008 Presidential general election worked really well, at a time when the whole nation was paying attention. Polling averages correctly predicted the winner of every state except Indiana, which was close. So the polls were right.
But a funny thing happens if you go back and look carefully - the polling wasn't quite perfect... sometimes, it was off by quite a bit... and there's a pattern.
We'll start with the heaviest polled states, those with 10 or more polls. Polls from the final 10 days of the election were included in averages if a trend was evident; otherwise, polls from October 1st onward were included. Research 2000 polls were excluded. The polling margins were off by anywhere from three points in favor of Obama to three points in favor of McCain. Really, quite good predictions. But look what happens if you plot it against percent Obama:
The horizontal black line indicates perfection. This is what all polls are aiming for. If Obama's margin was larger than predicted, the point falls above this line; if smaller, it is below. In other words, above the line the election results favored Obama compared to the polls, below the line the election results favored McCain when compared to the polls.
What we see is no random error. Excluding NH, there's a lovely correlation. And if you read the previous post in this series, this is a familiar looking graph...
So what does this mean for 2012? Polls will very likely underestimate Obama's performance in deep blue states, while underestimating Romney in deep red states. So don't worry if you see Obama polling at only 50% in Massachusetts. But in frequently polled states where Obama and Romney are nearly tied, the polling averages should be within a few points of the real vote margin.