The Gallup Poll's mid-September survey showing a huge Bush lead was based on interviews the firm conducted with 1,022 American adults over the course of three days. The main question Gallup asked was, "Suppose that the presidential election were being held today, and it included John Kerry and John Edwards as the Democratic candidates, and George W. Bush and Dick Cheney as the Republican candidates. Would you vote for John Kerry and John Edwards, the Democrats, or George W. Bush and Dick Cheney, the Republicans?" (The order of the candidates' names was varied for each interview.)

Although it was later discovered that Gallup's poll included a large number of Republicans, Gallup did not choose to call people it already knew were Republicans, Democrats or independents -- a common misconception online. Instead, in conducting election polls, Gallup and other firms go to a great deal of trouble to ensure that the people they interview are chosen completely randomly. Theoretically, the greater the randomness with which the sample is selected, the closer the sample will approximate the views of the larger population. To get this random sample, Gallup dials phone numbers constructed by a random-digit dialing machine, thereby including unlisted phones in its survey. Polling firms repeatedly call numbers that are busy or unresponsive. Still, despite such efforts, not many people cooperate with pollsters. A typical response rate -- the number of interviews conducted out of all the phone numbers attempted -- is under 30 percent, a statistic that has been declining over the past two decades. In order to reach 1,022 people for its poll, Gallup had to call thousands more who didn't respond or who refused to take part.

When it gets people on the phone, Gallup peppers them with questions about the election -- but although they interview all the respondents, pollsters know that not all of the people contacted will make it to the polls on Election Day. Since you only want to know which candidate is preferred by the people who will vote, polling firms need to guess which respondents will cast a ballot -- and this turns out to be one of the trickiest things in election polling. "There's no single question you can ask people that will capture the future fact" of whether they will vote, says Michael Dimock, research director at the Pew Research Center for the People and the Press. "We ask people flat out, 'Do you plan to vote?' and 95 percent of the people say they do. But we know that's not the case -- we don't get 95 percent of the people voting on Election Day."

Different polling organizations use different methods for sorting likely voters from unlikely voters, but Gallup's method is generally acknowledged to be more elaborate than that of other polls. Gallup says that the system is designed to increase the accuracy of the poll; critics, though, charge that the likely-voter methodology gives some candidates an unfair edge in the results.

Gallup attempts to gauge a respondent's likelihood of voting through a series of questions about an impending election. For instance, "How much thought have you given to the upcoming election for president -- quite a lot, or only a little?" Or, " Do you happen to know where people who live in your neighborhood go to vote?" "How often would you say you vote -- always, nearly always, part of the time, or seldom?" "In the last presidential election, did you vote for [candidate name] or [candidate name], or did things come up to keep you from voting?"

Gallup asks seven such questions; respondents who answer "correctly" on all of them are deemed likely voters. Of the 1,022 people interviewed for Gallup's mid-September poll, 767 of them -- about 75 percent -- made it through this likely-voter screen, and it was among this smaller sample of respondents that Bush beat Kerry by a 13-point margin. But it's important to note what Gallup means by the term "likely voter." To Gallup, the 767 respondents who answered its likely-voter questions correctly are not necessarily going to vote in the November election. Rather, the firm is saying that if an election were held today, it's these 767 people -- and the tens of millions in the population they represent -- who are most likely to go to the polls.

This is a subtle point, one not frequently explained in media coverage of poll results. Pollsters are not psychics; when they issue their election surveys from on high, they are not telling us what will happen tomorrow or three weeks or three months from now. Instead, and far less usefully, they are telling us what the public mood is today. Frank Newport, editor in chief of the Gallup Poll, compares preelection polling results to football scores taken during the course of a game. Obviously, the scores can be useful. A third-quarter score tells you something -- which team is doing better, which team is winning so far. But as a predictor of the final result, the third-quarter score is not perfect, for one obvious reason: Things can change. In a political campaign, as on the football field, winners can become losers, or vice versa, dramatically, quickly. A team can squander its momentum or, out of nowhere, come from behind with a winning play.

Presidential debates, like passes on the football field, can be fumbled. Speaking to Salon last week before John Kerry met George Bush in the first debate, Newport cited the 1980 race between Jimmy Carter and Ronald Reagan. Prior to the first debate that year, Carter was polling ahead. "But Reagan had a great debate performance," Newport recounts, "and then the Iran hostage anniversary came in, and in the end Reagan won by 10." Newport added: "With three debates still coming in this election, we wouldn't want to predict what's going to happen on Election Day."

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