The Accuracy of the Final National Polls

by John Sides on November 7, 2012 · 9 comments

in Campaigns and elections

Fordham University political scientist Costas Panagopoulos has compared the last pre-election national poll from each of the pollsters available in the Pollster data to the estimated national popular vote.  He writes:

For all the derision directed toward pre-election polling, the final poll estimates were not far off from the actual nationwide vote shares for the two candidates. On average, preelection polls from 28 public polling organizations projected a Democratic advantage of 1.1 percentage points on Election Day, which is only about 1 point away from the current estimate of a 2.2-point Obama margin in the national popular vote (Obama 50.3% versus Romney 48.1%).
Following the procedures proposed by Martin, Traugott and Kennedy to assess poll accuracy, I analyze poll estimates from these 28 polling organizations. Most (22) polls overestimated Romney support, although some only slightly, while 6 overestimated Obama support. Most (22) polls overestimated Romney support, while six (6) overestimated Obama strength, but none of the 28 national preelection polls I examined had a significant partisan bias.

I’ve plotted the data above a simpler quantity than what Panagopoulos calculated, which conveys the thrust of his analysis.  If the overall picture changes dramatically as the votes are counted, we will report back.  I also hope to have something on the state polls soon.

For more on this subject, see Mark Blumenthal, Harry Enten, and Simon Jackman.

(Errata: This post has been corrected to account for my misunderstanding of what Panagopoulos had done.  The data in my chart are not the basis of his analysis that I quoted above.)


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