Choosing Likely Voters

Why do polls talk about “likely voters”?

One of the values in the American political culture is that “a good citizen votes.” But like a number of these credos, more pay lip service to the idea than do so in their behaviors. So one problem election pollsters face is that not every respondent who tells them they plan to vote will actually do so. Actual turnout (known only after the election) is generally lower than respondents’ self-report of voting intentions in pre-election polls. So the pollsters’ dilemma here is to separate the wheat from the chaff: Of all those saying they will vote on Election Day, which ones will really do it, and which ones will stay home?

Presidential Election Years Turnout

Source: The United States Election Project. The voting-eligible population is constructed by adjusting the voting-age population for non-citizens and ineligible felons, depending on state law.  National estimates are further adjusted for overseas eligible voters.

Who are likely voters, and how are they determined?

Pollsters often address the problem of likely voters by giving each of their respondents a score, based on their answers to a number of questions that have been shown to be related to actually going to vote. They then establish a cutoff, based on what they think turnout is likely to be (say 50% of registered voters) and then describe the 50% with the highest score on their question index as “likely voters.” But even while most use such a scale, the component questions that go into the scale differ, and so this too is a reason why some election polls report different findings.

Research finds no magic bullet question or set of questions that can reliably determine likely voters with 100% accuracy. Thus, different organizations have different ways of identifying who they believe likely voters are. Most polls ask a combination of questions that cover self-reported vote intention, including measures of engagement (“Are you following the election closely?”) and past behavior (“Have you voted in prior elections?”).

A second issue in determining likely voters is estimating how many there will be, which may affect the division of the vote. In Minnesota, for example, the percentage of registered voters turning out in the last three presidential elections was 67% in 1996, 70% in 2000, and 77% in 2004. Based on this, a pollster might expect turnout to be at least 77%  in 2008. But, by comparison, surveys have noted that interest in the election is higher in 2008 than in prior years.

So, what should be used as the expected turnout figure, 77% or 80%, or 85%? Suppose a choice of a cutoff point of 80% gives an estimate that the Democrat leads the Republican in the state by 4 percentage points. But when expanding the expected electorate to 85%, it may be that the data show the Democrat leading by 6 percentage points. So, another source of possible differences is whether the level of support for each candidate changes as the “likely” electorate gets smaller or larger – or it doesn’t make any difference. Moreover, some may start with a base of registered voters (85% in Minnesota in 2004) while others may work with a percentage of the voting age population (72%) as their base. This means that the definition of a likely voter is somewhat of a moving target, compared to the definition of registered voters.

How much faith should I put in pollsters’ reports of likely voters?

There is a consensus in the polling community that it is better to report “likely” voters than “registered” voters, which is why survey organizations do it. But having acknowledged this, there are some cautions:

  • First, people change in their commitment to voting as the campaign unfolds. Respondents are probably better able to tell if they really are going to vote as it gets closer to Election Day. One of the reasons the polls get in closer agreement as Election Day nears is that all of their “likely voter” formulations behave more similarly. It is easier to identify true “likely voters” the closer it is to Election Day.
  • Second, there is no magic formula for identifying likely voters. There are a number of indicators, but the combination of them is a bit subjective between polling organizations. As of now, there’s no clear right or wrong formula.
  • Third, no one knows what actual turnout is going to be. It’s like trying to hit a moving target. A polling organization might make a prediction based on a turnout of 70% of registered voters (that they designate as “likely”) but, there are high and low interest elections. The past is not a perfect guide to the future. No one knows what’s actually going to happen. That’s why we have the elections.

So, pre-election polls of likely voters are “best estimates.” Good ones, as the history of numerous polling organizations over numerous years have shown, but just best estimates nonetheless. However, it’s always hard to argue against proper caution and respect for nuance in reporting on these polls.

This information was developed by AAPOR as part of a comprehensive online journalism polling course created in partnership with NewsU, a project of the Poynter Institute and funded by the Knight Foundation. The course launched  September 2007.