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Obama’s bounce in state vs national polls

Particularly before the Newsweek poll came out last night, which showed an oversized, 15-point post-primary bounce for Barack Obama, there had been an apparent discrepancy in the size of the bounce that Obama had received in state and national polling.

Firstly, let’s update our chart of Obama’s numbers in state polling. What follows is a comparison of the Obama less McCain margin in all states that have been polled since the primaries concluded that were also surveyed by the same agency at some point in May:

State / Agency      May     June    Change
AR Rasmussen -24 -9 +15
KY SurveyUSA -24 -12 +12
OH Quinnipiac -4 +6 +10
KS Rasmussen -19 -10 +9
ME Rasmsseun +13 +22 +9

GA Insider Adv -10 -1 +9
FL Quinnipiac -4 +4 +8
NY Siena +11 +18 +7
WA Rasmussen +11 +18 +7
NH Rasmussen +5 +11 +6

PA Quinnipaic +6 +12 +6
WI Rasmussen -4 +2 +6
AK Rasmussen -9 -4 +5
IA Rasmussen +2 +7 +5
CA SurveyUSA +8 +12 +4

VA Rasmussen -3 +1 +4
MI Rasmussen -1 +3 +4
WA SurveyUSA +14.0* +17 +3
NV Rasmussen -6 -3 +3
WI SurveyUSA +6 +9 +3

FL Rasmussen -10 -8 +2
NC Rasmussen -3 -2 +1
NC Civitas -5 -4 +1
OH Rasmussen -1 -1 0
MN Rasmussen +15 +13 -2

MN SurveyUSA +5 +1 -4
CO Rasmussen +6 +2 -4
IA SurveyUSA +9 +4 -5
OR Rasmussen +14 +8 -6
AVERAGE -0.1 +4.0 +4.1

* Average of all May surveys.

Across 29 state polls, Obama’s bounce is 4.1 points — down slightly from our estimate earlier in the week. The bounce appears to be roughly normally distributed; if you drew a histogram of the bounce in individual states, it would resemble a bell curve.

What about Obama’s bounce in national polling?

State / Agency      May     June    Change
US Newsweek 0 +15 +15
US Harris +4 +11 +7
US Economist -3.2* +3 +6.2
US Rasmussen -1.1* +5 +6.1
US USA Today +2.0* +6 +4
US AP-Ipsos +4 +7 +3
US Cook/RT +1 +4 +3
US Gallup +0.1* +2 +1.9
US ABC/WaPo +7 +6 -1
US Diageo/Hotline +4 +2 -2
US Zogby +8 +5 -3
US IBD/Tipp +11 +3 -8
AVERAGE +3.1 +5.8 +2.7

* Average of all May surveys.

In national polling, Obama’s average bounce has been 2.7 points. That isn’t all that far away from our state polling bounce to begin with, but there are a couple of additional things to notice that make discrepancy even smaller.

Firstly, the national polls that showed Obama regressing this month all had him at a pretty high number before. ABC/WaPo had him 7 points ahead of John McCain last month, Zogby had him 8 points ahead, and IBD/Tipp had him 11 points ahead. The most recent version of each of these polls all showed Obama regressing. But really all Obama may have been doing is regressing toward the mean. Does anybody really believe that Obama was ahead by 7, 8 or even 11 points last month? That’s certainly not the impression one was getting from the state-by-state polling results we were seeing in May. If Obama wins this election by 11 points, you will see things happen like him winning Pennsylvania by 16 points, or winning states like Texas, Arizona and Kansas, or winning Florida by high single digits. Those aren’t the sorts of results we are seeing now, and they certainly weren’t the sorts of results we were seeing in May.

So let’s instead focus only on those pollsters that release national polls on a weekly-or-better basis. This means Rasmussen and Gallup, which release numbers daily, and Economist/YouGov, which releases numbers weekly. These polls are going to be far less subject to problems with small sample sizes than surveys that are conducted just once a month.

State / Agency      May     June    Change
US Economist -3.2* +3 +6.7
US Rasmussen -1.1* +5 +6.1
US Gallup +0.1* +2 +1.9
AVERAGE -1.4 +3.3 +4.7

* Average of all May surveys.

These three polls show an average bounce of 4.7 points, barely different from our finding in the state-by-state results. If you exclude the Economist’s poll and focus only on the two daily trackers, Obama’s present bounce is 4.0 points, which is even closer to the target established by the state polls.

To the extent that one is going to use national polling results to divine trends, one ought to give a relatively large weight to the two national tracking polls. Gallup and Rasmussen are each surveying about 25,000 voters per month for their national trackers, as compared to a once-a-month survey like ABC/WaPo, which might poll one-twentieth that many people. Although there are some diminishing returns associated with large sample sizes, it is also not a case where all polls should be treated equally.

Nate Silver founded and was the editor in chief of FiveThirtyEight.