Trusting In The Polls

John Sides recommends it:

[T]here is the question of whether polling misses might mean that the Democrats end up with a slim Senate majority after all.

There are reasons to be skeptical that this will happen.  It’s not just that we can’t easily predict whether the polls will over- or under-estimate one party’s vote share, as discussed by Nate Silver and by Mark Blumenthal & Co.  And it’s not just, as Josh Katz and Sean Trende have found, that Senate polls already tend to be pretty accurate at this point in time — especially when candidates have a 3- to 4-point lead, as do Republican candidates in Alaska, Arkansas, Georgia, Kentucky and Louisiana.

The other key point is this: Late movement in Senate polls tends to be in the direction of the underlying fundamentals.

Sean Trende agrees:

The biggest problem with these sort of data — very little variance, small number of observations — is that they invite introduction of our own biases. I don’t mean bias in the crudest sense of the term, although I don’t think it is accidental that the people discussing poll skew in 2012 tended to be conservatives, and vice-versa this year. … If you only have a dozen or so data points and go looking for a pattern, sooner or later you will find something that explains those data points well. The problem is that we don’t have a great basis for sorting out the good theory from the bad, at least until the theory has survived a few trial runs.

To see the potential problem here, when I look at close races only in midterms, the pattern that jumps out at me is that pollsters understate the “victorious” party. 1994 and 2002 were good Republican years, and there was a pro-Democratic bias. 1998 and 2006 were good Democratic years, and there was a pro-Republican bias. This might suggest that there will be a pro-Democratic skew this year.