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.