The WaPo-ABC ‘swing state’ poll numbers, explained

October 1, 2012

Monday’s Washington Post-ABC News poll adds to the evidence of an emerging, important dynamic in the presidential contest showing closer parity nationally than in key battleground states, where President Obama has had clear leads.

Three Washington Post polls over the past two weeks -- in Florida, Ohio and Virginia -- all showed Obama with an edge over Mitt Romney. Recent state polls from other public pollsters in Colorado, Iowa, New Hampshire, Nevada and Wisconsin mainly show the same thing: advantage Obama.

Pulling out respondents in these eight states -- all of which Obama won in 2008 -- from the new national poll, shows Obama at 52 percent and Romney at 41 among likely voters. We report these numbers to help connect the dots between the clear Obama leads in the collection of states and the continued closer parity nationally, not to suggest pinpoint precision on what’s happening now in any particular state.

Out of 929 registered voters in the new poll, 161 live in one of these eight states, with a margin of sampling error of eight points. The likely voter sample in these states is about nine points, making the 11-point gap an apparent edge. That margin is significant at the 80 percent confidence level, not a standard, conservative 95 percent threshold, which we take as added evidence of the state-of-play in state polling, and the crucial link between those and the national numbers (49 percent for Obama, 47 percent for Romney). No such dissection of a national poll -- no matter how many interviews -- could be anything more than that. They are simply not designed that way.

Focusing on the 11-point margin is mistaking the trees for the forest, which is why the paragraph where we use the numbers includes four surveys, not just the new one. (Here's the article.)

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Aaron Blake · October 1, 2012