The ins and outs of polling voters

Obama is galloping away with the presidential race. Or maybe he has a modest lead. Or maybe he and John McCain are neck and neck.

Confusing? Sure, thanks to the dueling results of recent major polls.

In the past week, most surveys have shown Democrat Obama with a significant national lead over Republican McCain. Focusing on “likely voters” — as many polling organizations prefer this close to Election Day — an ABC News-Washington Post survey showed Obama 11 points ahead. A Wall Street Journal-NBC News poll had the same margin, while the nonpartisan Pew Research Center gave Obama a 14-point edge.

But others had the race much closer. CNN-Opinion Research detected an Obama lead of 5 points. The George Washington University Battleground Poll had Obama up by 4 points. And an Associated Press-GfK poll showed Obama at 44 percent and McCain at 43 percent — in effect, a tie.

How can this be? Some questions and answers.

Q: Don't pollsters simply ask questions, tally the answers and report them?

No. After finishing their interviews — usually with about 1,000 people, sometimes more — they adjust the answers to make sure they reflect Census Bureau data on the population like gender, age, education and race. For example, if the proportion of women interviewed is smaller than their actual share of the country's population, their answers are given more “weight” to balance that out. But some pollsters make these adjustments differently than others.

Q: Are those the only changes?

No. As Election Day nears, polling organizations like to narrow their samples to people who say they are registered voters. They often narrow them further to those they consider likely voters.

Q: Is that hard to do?

Quite hard, since no one will truly know who will vote on Election Day until that day is over. In fact, virtually every polling organization has its own way of determining who likely voters are.

Like many polling organizations, the AP asks several questions about how often people have voted in the past and how likely they are to vote this year, and those who score highest are considered likely voters.

Q: Why is this such a problem?

Because nobody is 100 percent sure how to do this properly. And the challenge is being compounded this year because many think Obama's candidacy could spark higher turnout than usual from certain voters, including young people and minorities. The question pollsters face is whether, and how, to adjust their tests for likely voters to reflect this.

In identifying likely voters, the AP does not build in an assumption of higher turnout by blacks or young voters. Pew Director Andrew Kohut says that reflecting exceptionally heavy African-American turnout in the Democratic primaries, Pew's model of likely voters now shows blacks as 12 percent of voters, compared to 9 percent in 2004.

Q: What else might cause differences?

The groups pollsters randomly choose to interview are bound to differ from each other, and sometimes do so significantly.

Every poll has a margin of sampling error, usually around 3 percentage points for 1,000 people. That means the results of a poll of 1,000 people should fall within 3 points of the results you would expect had the pollster instead interviewed the entire population of the U.S. But — and this is important — the results are expected to be that accurate only 95 percent of the time. That means that one time in 20, pollsters expect to interview a group whose views are not that close to the overall population's views.

Q: Are people always willing to tell pollsters who they're supporting for president?

No, and that's another possible source of discrepancies. Some polling organizations gently prod people who initially say they're undecided for a presidential preference, others do it more vigorously. The AP's poll, for example, found 9 percent of likely voters were undecided, while the ABC-Post survey had 2 percent.