Too close to call

by Andrew Gelman on October 31, 2012 · 28 comments

in Campaigns and elections

From my online NYT column:

For months now people I know have been stopping me on the street to ask who I think will win the election. And for months I’ve always said, either side could win it. . . . Different models use different economic and political variables to predict the vote, but these predictions pretty much average to 50-50. . . .

Last week I was interviewed by a reporter from France, who asked me who I thought would win the election. I said, it’s too close to call. He said, but Nate Silver’s FiveThirtyEight blog on NYTimes.com currently gives Obama a 72.9 percent chance. I think Nate is great, and I was thrilled to have the opportunity to contribute to his blog for a while. But I’d still say the election is too close to call. . . .

The online betting service Intrade gives Obama a 62 percent chance of winning, and I respect this number too, as it reflects the opinions of people who are willing to put money on the line. The difference between 63 percent and 75 percent may sound like a lot, but it corresponds to something like a difference of half a percentage point in Obama’s forecast vote share. Put differently, a change in 0.5 percent in the forecast of Obama’s vote share corresponds to a change in a bit more than 10 percent in his probability of winning. Either way, the uncertainty is larger than the best guess at the vote margin.

Where is this uncertainty coming from? First, public opinion can shift quickly during the last week of campaigning, as news arrives and as voters make their final decisions. Second, the polls aren’t perfect. Nonresponse rates continue to rise and, although pollsters are working on more and more sophisticated methods for correcting for this problem — an area of my research — we cannot always catch up. Even an average of polls can be wrong, but it’s hard to know in advance of the election how wrong they will be, or in which direction.

What I’m saying is that I can simultaneously (a) accept that Obama has a 72 percent chance of winning and (b) say the election is too close to call. . . .

More at the link, including a discussion of why it’s not correct to say that “Romney’s a touchdown behind with five minutes to play.”

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