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Gallup's New Likely Voter Model

Topics: Charts , Gallup , Likely Voters , Pollster.com

Though I caught reference to it elsewhere, I managed to overlook the detailed description in today's Gallup Daily release of how they will report "likely voter" results for the rest of the campaign:

Likely Voter Estimates

Obama's current advantage is slightly less when estimating the preferences of likely voters, which Gallup will begin reporting on a regular basis between now and the election. Gallup is providing two likely voter estimates to take into account different turnout scenarios.

The first likely voter model is based on Gallup's traditional likely voter assumptions, which determine respondents' likelihood to vote based on how they answer questions about their current voting intention and past voting behavior. According to this model, Obama's advantage over McCain is 50% to 46% in Oct. 9-11 tracking data.

The second likely voter estimate is a variation on the traditional model, but is only based on respondents' current voting intention. This model would take into account increased voter registration this year and possibly higher turnout among groups that are traditionally less likely to vote, such as young adults and racial minorities (Gallup will continue to monitor and report on turnout indicators by subgroup between now and the election). According to this second likely voter model, Obama has a 51% to 45% lead over McCain.

With a fifty-year time series of presidential polling to consider, Gallup has often demonstrated a reluctance to change its methods. As such, Gallup does deserve credit for trying, as Jay Carney put it today, "to apply a [new] model that accounts for the electorate's likely new complexion," even if they are essentially "hedging their bets by going with two models."

Of course, that hedging presents us with a difficult decision to make about which Gallup results to include in our national trend charts. Our usual rule is to give preference to results among registered voters over samples of adults, and to "likely voter" samples over registered voters. Charles Franklin and I had a two-part exchange on this subject back in August that explains the rationale for our usual rule. As Franklin put it, "our first rule for Pollster is that we don't cherry pick." So we rely on a simple inclusion rule that relies on the pollsters' judgements:

Our decision rule says "trust the pollster" to make the best call their professional skills can make. It might not be the one we would make, but that's why the pollster is getting the big bucks. And our rule puts responsibility squarely on the pollsters shoulders as well, which is where it should be.

Unfortunately, in this case, Gallup is producing two different "likely voter" models without expressing a clear preference for either. So in this rare case, we will exercise our own judgement and opting to plot Gallup's newer "Likely Voter Model II," at least for the time being. Why? First, the Likely Voter II (the one based only on respondents current voting intention) splits the difference between the registered voter results we have been reporting for the Gallup Daily and the traditional model.

Second, and more important, the traditional Gallup likely voter model has been producing samples that have significantly fewer 18-to-29-year-olds than both the likely voter models of other pollsters and available estimates of the 2004 electorate. While no one can be certain about who will vote, the least likely outcome is a 2008 electorate that is older than those who voted in 2004.

Now should Gallup change and express a clear preference for either model, we will yield to their judgement. Until then, we will plot the "likely voter II" model for both Gallup Daily (as of today) and the remaining USA Today/Gallup polls.

Update: Nate Silver comes to the same conclusion. 

 

Comments
falcon79:

Excellent call!! :)

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thoughtful:

I concur

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s.b.:

So you choose to give Obama an arbitraray 3% advantage that goes against 50 years of Gallup's polling history and your OWN rule of using the strictest screen from any given poll. Wow! What else is there to say. Riots in the streets anyone when the bias of the pollsters turns out to be wrong? Let's hope not but one assumes there is bias in the methodology as well at this point if they won't even report their own 50 year trusted and proven results. I am very disappointed in pollster. Will the riots in the streets be a result of racism or just media and pollster bias trying to throw an election? Perhaps you can poll on that after it happens.

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s.b.:

Do you suppose any of the other polls aren't appropriately screening their respondants and that the Obama campaign has gotten to them to? How about all those New Democratic registrations up to 50% of which are turning out to be fraudulent? So when only about 65% of people vote to you suppose a poll where 95% of respondants is accurate? Also all of the tighter screens have shown a smaller spread and none of them even screen to get to 65%. So lets say voter turn out is historic and goes up to 70-72% turnout. That's still 13-15% of people that shouldn't be included in these polls. How much does that swing the actual voting day results to McCain. It is absolutely wreckless, irresponsible and in a small way fraudulent to not be reporting the strictest screen available in polls. And you know it.

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JFactor:

It's a judgement call and I don't envy those who has to make it. You will get flack no matter which way you go. Gallup is ineed hedging its bets and that's ultimately pretty lame.
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http://www.internationalpoliticstoday.com

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RS:

What JFactor said.

@s.b. - did you read the part where Mark B. says "the LV-II model splits the difference"?
So Mark B. could have actually used the model that gives Obama a bigger margin, but he didn't. I also think he gave a pretty convincing explanation why the Gallup LV-I model might not be the best way of doing things.

Of course, I'd prefer Mark B. only uses polls that show the race really tight - that avoids complacence and helps GOTV... :-)

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Jimbo77:

6 to 1 obama margin in the included respondents vs those excluded in the traditional LV model. That is very unlikely. RCP is weighting each as half so you end with one data point on there chart.

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