Silver on Polling Bias

Nate Silver has one of those posts today at FiveThirtyEight you feel like you should memorize, since it covers a lot of the misunderstandings and arguments left in this midterm election cycle.

First he takes on this year’s version of the “skewed polls” controversy of 2012, and reminds us that as it turned out the 2012 polls were generally off–but in favor of Republicans.

It’s a bit of a shock to read Nate’s data and realize that Senate polling averages in the last three weeks of the campaign have been off by more than 3% four times since 1990: twice showing “bias” towards Democrats (1994 and 2002) and twice towards Republicans (1998 and 2012). Bottom line:

On average since 1990, the average bias has been just 0.4 percentage points (in the direction of Republicans), and the median bias has been exactly zero.

Not much predictive value there.

How about turnout? Could the polls be missing the hidden effect of, say, the Brannock Street Project? Maybe, but they’re already showing a narrowed gap between registered and likely voters, a good sign for Democrats:

[T]he pollsters, at least as a group, are not expecting the sort of turnout gap they did in 2010. That year, the average poll had Republicans doing about 6 percentage points better among likely voters than among registered voters — a historically large difference. The average poll we’ve tracked this year has shown about a 3-point gap (favoring Republicans) instead — in line with the historical average in midterm years.

And remember, the question is not which party has the stronger ground game, but whether a stronger ground game will lead to benefits that aren’t reflected in the polls.

In passing, Nate also reminds us of election theories you still hear but that have been largely discredited: the Incumbent Rule (undecideds break towards challengers); the Bradley Effect (polls overestimate the vote of African-American candidates); and the Generic Ballot Tilt (the generic congressional ballot has a built-in Democratic bias).

But the biggest takeaway is that very small changes in the national climate could have profound effects:

Suppose the polls have just a 1 percentage-point Republican bias, for example, in the average Senate race. In that case, Iowa and Colorado would go from slightly favoring Republicans to being more like true tossups. Democrats would also be slightly more likely to pull off an upset in a state such as Georgia, where the GOP has only a tenuous advantage. Overall, the probabilities for Senate control would come out to almost exactly 50-50.

However, the Democrats’ path would become much steeper if there’s even a modest Democratic bias in the polls. As the race stands, they’ll need to win a couple of states where they are slight underdogs. If they become somewhat heavier underdogs in those states, their chances of getting “lucky” in two or more states on election night will diminish toward zero. According to our model, Republicans would be 92 percent favorites to take the Senate if the polls have a 1-point Democratic bias, and all but certain to do so if it’s any larger than that.

Considering the massive spin we’re going to hear no matter what happens on November 4, that’s worth remembering. This is an election where Republicans have extraordinary built-in advantages, at least when it comes to Senate races. But it’s very unlikely we’re going to see a “wave election” (unless you just define that as a certain number of “wins” as opposed to a big final surge in popular votes) or some sort of big pro-Democratic surprise. Still, the range of possibilities is pretty broad, so we’re still on tap at present for an interesting election night and quite possibly Overtime (runoffs), Double Overtime (two runoffs), or even Triple Overtime (two runoffs plus an indie or two mulling party options).

Ed Kilgore

Ed Kilgore, a Monthly contributing editor, is a columnist for the Daily Intelligencer, New York magazine’s politics blog, and the managing editor for the Democratic Strategist.