NH: The Gallup "Likely Voter Model" Arrives

12/21/2007 08:16 pm ET | Updated May 25, 2011

One quick note about the new Gallup poll from New Hampshire that we linked to a few moments ago (see Gallup's releases on the Democratic & Republican samples). As the Gallup release indicates, it is based on their well-known but sometimes controversial "likely voter model." That fact alone makes their results different -- and not entirely comparable -- to the other New Hampshire polls we have seen.

To the extent that they have disclosed their methods, the other polls we have reported on typically use some sort of screen: They include self-reported registered voters who indicate some degree of intent to vote in either the Democratic or Republican primary. The Gallup model, whether applied here or in a general election, builds on the idea that self-reported intent to participate alone tends to overstate the true turnout. So they use other measures that tend to correlate with turnout, such as attention paid the the campaign, self-reported voting in past elections, and knowledge of the location of their polling place, to narrow the sample to a percentage approximating the likely turnout. The new survey from New Hampshire applies exactly that model.

I emailed Gallup and they kindly provided this detailed document describing the mechanics. The model uses eight questions to build an eight-point scale, on which a score of eight indicates the highest probability of voting. In the current survey, they used the following questions and awarded respondents with one point for every bolded answer below:

1A. Generally speaking, how much interest would you say you have in politics -- a great deal, a fair amount, or only a little?

1B. How often would you say you vote -- always, nearly always, part of the time, or seldom?

1C. Do you happen to know where people who live in your neighborhood go to vote? (Yes or no)

1D. Have you ever voted in the polling place or ward where you now live? (Yes or no)

1E. How much thought have you given to the coming primary election for president 8211 a great deal, a moderate amount, not much, or none at all?

D8. Next, I'd like you to rate your chances of voting in the primary election for president on a scale of ten to one. If '10' represents a person who definitely will vote and '1' represents a person who definitely will not vote, where on this scale of ten to one would you place yourself? (7-10 or 1-6)

D9. Thinking back to the election in November of 2006 when John Lynch ran against Jim Coburn for governor of New Hampshire did things come up that kept you from voting, or did you happen to vote in that election? (Yes or no)

D10. Please tell me whether you, yourself, ever voted in each of the following kinds of elections. How about...

A. A Republican or Democratic primary for president

B. A Republican or Democratic primary for U.S. Senator or Congressman

C. A Republican or Democratic primary for Governor

(A yes on any earns a point).

Because of the built in penalty for those who were not old enough to have voted in previous elections Gallup gives extra points to those age 18-21. They also give an extra point to those who did not live in New Hampshire in 2006 but say they "always" or "nearly always" vote. [Clarification: Since Marc Ambinder quoted this paragraph, it made me (a) notice the typo, now corrected and (b) want to point out that 18-21 year olds get an extra point or two to the degree that they score high on the other questions. The point is that those otherwise earn a perfect likelihood score are not penalized because they were not old enough to have voted before].

I'm oversimplifying a little here (see their description for full details), but Gallup then selects some combination of respondents scoring 6 or higher weighted so that their sample size matches the expected turnout. Here is how Gallup explains their assumptions about New Hampshire turnout:

Turnout has been fairly high in recent primaries, roughly 20-25% of New Hampshire adults have voted in the Democratic primary and 25% have voted in the Republican Primary.

Given the usual incidence in our polls of 40% of New Hampshire adults saying they will vote in the Democratic primary and 40% saying they will vote in the Republican primary, the typical turnout assumptions are that 50%-55% of self-reported Democratic primary voters and 60% of self-reported Republican primary voters should turnout (roughly half of New Hampshire residents). . . Given a higher proportion of Democrats scoring as likely voters this year than in previous years, the expected turnout for Democrats was increased to 60%.

I wrote at great length during the 2004 campaign about the Gallup likely voter model and the shortcomings identified by critics. The short version is that while this model produces a lot of questionable variation when applied months before an election, as well as results that differ from other likely voter models, its track record is strong when applied the the last survey before the election.

The even shorter version: The Gallup model may produce different results than other recent polls in New Hampshire. Debate which model makes the most sense -- and I know you will -- but be careful about comparisons across polls.