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Sept. 13: After Convention Bounce, Holding Obama’s Polls to a Higher Standard

On Thursday at 6:30 p.m. Eastern time, there was a deluge of comments in my Twitter feed when NBC News and Marist University released poll results showing President Obama with clear leads in three swing states. Mr. Obama led by five points in their polls of Virginia and Florida, and by seven in their poll of Ohio.

These are undoubtedly good surveys for Mr. Obama, and they came in crucial states. And yet, our forecast model did not move toward Mr. Obama on Thursday. Instead, Mr. Obama’s forecast declined slightly; the model now gives him a 78.6 percent chance of winning the Electoral College, versus 80.7 percent on Wednesday.

Part of this is because the other polling data released on Thursday was not as strong for Mr. Obama as the set of NBC-Marist polls, but let me make a broader point first.

We have seen a shift toward Mr. Obama in the polls since the Democratic convention. It appears that if an election were held today, he’d win it by somewhere in the neighborhood of four or perhaps five percentage points.

If Mr. Obama is ahead by four to five points nationally, we’d certainly also expect him to post his share of leads by about that margin in swing states. Because of statistical variance and differences in methodology, some of the numbers are going to be a little bit better for him than others. But the consensus of the data ought to quite strong for him.

The Marist polls probably did meet that standard. But there were also two other polls of Ohio released on Thursday that showed Mr. Obama up only one point instead, along with a trio of Florida polls showing a tie there, on average. Those aren’t bad numbers for Mr. Obama exactly, but they aren’t great ones either — they are more like those we were seeing from the polling firms in question before the conventions.

There were also polls out on Thursday in several other swing states — in New Hampshire, Michigan and Colorado, for example. The data, taken as a whole, was pretty good for Mr. Obama, as he led in almost all of the surveys, although mostly by small margins.

But the forecast model is now judging Mr. Obama by a higher standard. Why? Because it had more or less fully priced in his convention bounce as of a few days ago. In fact, its assumption is that Mr. Obama’s polls probably slightly exaggerate his standing right now.

To meet the “new normal” that the model expects, it won’t be enough for Mr. Obama to have pretty good polls. He’ll need to have his share of very good ones, like the Marist polls.

After another week or two, the pressure will be relieved on Mr. Obama somewhat, since at that point it will become safer to conclude that whatever bounce he received after his convention may be permanent.

But if Mr. Obama’s state polling results in the interim are merely pretty good, the model may revert to having a merely pretty good forecast for him — meaning, more like a 70 percent chance of winning re-election, which is about where it had him before the conventions, rather than closer to 80 percent, as it now shows.

We’re not going to be lacking for data to make these inferences; the number of polls is increasing at a rapid rate. That also means, though, that there’s even less reason than before to focus on just one or two polls at the expense of the consensus of the data.

I’m not necessarily saying that all polls should be weighted equally (nor does the model do so). And it is mildly interesting that Mr. Obama’s bounce seems to have been larger in some of the higher-profile surveys (like those from Marist, Gallup, and CNN) that use a more thorough methodology.

But we’ve suddenly gone from having perhaps two state polls released every day to more like a dozen. (There has also been an increase in the number of national surveys.) When this happens, there can be the tendency for the news media to focus on those polls that confirm its current narrative about the race, while ignoring those that might tell a different story.

The state polls are more newsworthy, I suppose, if you had been skeptical about the notion that Mr. Obama had gotten a bounce, and needed further confirmation. Mr. Obama’s probability of winning has increased over the last few days at betting markets like Intrade and as according to bookmakers. There was not much reason to doubt that Mr. Obama had gotten a bounce, however, so this may simply mean the conventional wisdom was a little slow to catch up.

Nate Silver is the founder and editor in chief of FiveThirtyEight.