Polls, of course, get more accurate as predictors, the closer you get to the election. Even a month out, it is still a risky game to make predictions, but the picture, as Dale described, is getting much clearer.
I heard a talk by a political scientist last year who was looking at the accuracy of the polls compared to the Iowa Election Market. has anyone looked at IEM lately? He found that if you apply a correction to the polls that is based on previous historical relationships between polls a certain number of days out from the election, you can actually get a much more accurate prediction for the election.
Of course, to do this, you need a lot of data, and you need to run some tedious statistical analyses, and I don’t know if he’s doing that this year. If he is, I’m sure it’s out there on the net.
My personal sense is that a month out, it’s all but over. Although several things can go wrong. The polls might be under-estimating the Republican support, as they did in 2000. They might be over-estimating Obama’s support, because of the lying factor (people say they’ll vote for a black candidate when they really won’t because they don’t want to apear prejudiced). They might be under-estimating the Obama vote, because of his registration efforts.
Usually “likely” voters are polled, because who cares about the opinions of people who don’t vote? But likely voters are often defined as those who have voted in the last election (presidential, or congressional, depending on pollster), so it misses those who have just registered.
As to “undecideds”? They baffle me. But I believe it is true that some decide at the last minute. Usually they swing the way other folks are swinging.
In understanding why poll results change, it helps to understand statistics. Each poll is based on a sample of the overall population. When you randomly sample, there’s always a chance you happened to select a sample that does not accurately reflect the entire population. Pollster’s rarely report their estimates of error. We just get the straight poll results, without confidence intervals (how far off the estimates might be).
If you draw ten samples from the same population, you expect them to say different things. If you average the polls together, you well hone in on the population preference. That’s why I like looking at the Poll of polls. It’s also fun to look at the Iowa Election Market.
The “winner take all” market is showing that people have to pay around 75 cents per vote for Obama, and you can have McCain votes for only 25 cents. In the vote share market, it seems that folks believe Obama will get around 54% of the total vote, compared to McCain’s 46%. Hmmm. I should go take Obama at that price!