Nate Silver: The Polls Were Skewed Towards Democrats

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For much of this election cycle, Democrats complained the polls were biased against them. They said the polls were failing to represent enough minority voters and applying overly restrictive likely-voter screens. They claimed early-voting data was proving the polls wrong. They cited the fact that polls were biased against Democrats in 2012.

The Democrats’ complaints may have been more sophisticated-seeming than the ”skewed polls” arguments made by Republicans in 2012. But in the end, they were just as wrong. The polls did have a strong bias this year — but it was toward Democrats and not against them.

Based on results as reported through early Wednesday morning — I’ll detail our method for calculating this in a moment — the average Senate poll conducted in the final three weeks of this year’s campaign overestimated the Democrat’s performance by 4 percentage points. The average gubernatorial poll was just as bad, also overestimating the Democrat’s performance by 4 points.

The problem with Democrats’ claims is that they were one-sided. I don’t mean that in the typical false equivalence way. I mean that they were ignoring some important empirical evidence. This evidence suggests that polling bias has been largely unpredictable from election to election. Beyond the shadow of a doubt, the polling was biased against Democrats in 1998, 2006 and 2012. However, just as certainly, it was biased against Republicans in 1994, 2002 and now 2014. It can be dangerous to apply the “lessons” from one election cycle to the next one.

http://fivethirtyeight.com/features/the-polls-were-skewed-toward-democrats/