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How Does Gallup Define "Likely Voters"?

by Frank Newport and Colleen Sullivan


As has been well documented, voter turnout in Great Britain has declined steadily over the past half a century. Indeed, in the general election of 1997 only 71% of the eligible British voting age population actually cast a vote on polling day. Likewise, turnout in the June 1999 European Parliament elections was an abysmal 23%. A recent Gallup Poll has predicted that voter turnout in the general election scheduled for June 7th may be the lowest since the 1918 election.

Given these trends of increased voter apathy and low turnout, the voting intention results that Gallup produces from now until June 7th are based on the subset of eligible voters identified as most likely to actually cast a ballot. This subset often has different characteristics than the total population. Thus, the results of a typical election are different than they would be if every citizen actually turned out and voted -- because the people who actually vote are different from the people who don't. If pre-election polls don't take this into account, they run the risk of estimating an election result that will differ from the actual vote on polling day.

Gallup pollsters in the United States have devoted more than 60 years of work to perfect the process of scientifically determining ahead of time who is most likely to turn out and vote on Election Day. The basic process attempts to winnow down the total adult population in the weeks prior to an election to those most likely to vote -- or in other words, likely voters.

The challenge for the pollster is to find the best procedures or mechanisms in order to estimate who will be in and who will be out of the population of voters, and then to find a way of sampling opinion just from those who have the highest likelihood of voting.

The first and easiest step for pollsters is screening for specific, known voting requirements. Gallup begins with a random sample of the general population and simply asks them, once they are reached on the telephone, whether or not they are on the electoral register. Typically, more than 95% of the general population report that they are on the current electoral register. Thus, as a starting point, Gallup eliminates less than 5% of those we contact from our pool of likely voters.

Beyond that point, some pollsters have historically used background or demographic characteristics to determine likely voters, eliminating from a sample, for example, younger voters or those with lower levels of education. But, the impact of such background characteristics varies from election to election. In some elections, specific factors may activate or excite certain groups of voters and cause them to vote at higher proportions than their representation in the general pool of registered voters might predict. For instance, an election that has hotly debated pension-related issues might activate older voters.

Therefore, Gallup generally uses the views, opinions and self-perceptions of the voters themselves as a mechanism for isolating likely voters. The simplest example of such a "likely voter" question is the most straightforward: "Are you going to vote on Election Day?" A pollster could simply accept the respondents at their own word, and include in the final likely voter sample those who say "yes," and eliminate those who say "no." The major problem with this procedure is the natural inclination of the majority of registered voters who are interviewed before an election to say that they are going to vote -- without giving it much thought.

But, variants on the idea of asking individuals questions in order to determine their probability of falling into the sample of likely Election Day voters can be quite effective. Over the years, Gallup has developed a battery of questions that provide a good prediction of the probability that an individual will end up voting. These questions include asking whether or not the individual voted in the last election, how closely the person is following the election, how often they vote in elections, and so forth.

Based on more than 65 years of experience predicting "likely voters," Gallup's analysts have developed computer programs that give each person interviewed a score based on how they answer these questions. Quite simply, those with the highest score have the highest probability of voting, while those with low scores have a low probability of voting.

At this point, the process Gallup uses reduces the total sample (which represents the population of eligible voters) down to a size that best estimates the percentage of the voting age population that will vote on Election Day. If turnout is estimated to be roughly 70%, a sample of 1,000 is reduced to about 700, throwing out those respondents who have the lowest "likely voter" scores. In short, the procedure involves mimicking the real-world process by developing a final "likely voter" sample that approximates the ratio of likely voters to the total adult population in the real world.

It is interesting to note that so far the voting intentions of likely voters are not significantly different from the voting intentions of those who are less likely to vote. However, this may change in the days leading up to June 7th.

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