So we’re now down to the lick-log in the midterm elections, and some observers are spinning the latest polls to predict Total Victory for The Team, and some are arguing the polls are wrong. Nate Silver comes along at FiveThirtyEight to provide an empirical take on whether and how the polls may be wrong, and I distilled his wisdom at Washington Monthly:
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).
The bottom line is that past experience doesn’t tell us much about the likely accuracy or inaccuracy of polls this year. What we do know is that the landscape, particularly for the Senate, is skewed heavily in favor of the GOP, and that Democrats are fighting impressively to overcome a lot of built-in obstacles. How much they need to overcome and whether they succeed is something we won’t know until November 4.