- The Washington Times - Tuesday, November 4, 2008

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

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

Some questions and answers about why the polls differ.

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 such as 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. Some pollsters make these adjustments differently than others - and while most polling organizations, including the Associated Press, do not modify the responses to reflect some recent tally of how many Democrats, Republicans and independents there are, some do.

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. That’s because in a country where barely more than half of eligible voters usually show up for presidential elections, pollsters want their polls to reflect the views of those likeliest to vote.

Quite hard, because 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 are likely voters.

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

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

In identifying likely voters, AP does not build in an assumption of higher turnout by blacks or young voters. Andrew Kohut, director of the Pew Research Center for the People & the Press, says that reflecting exceptionally heavy black turnout in the Democratic primaries, Pew’s model of likely voters shows blacks as 12 percent of voters, compared to 9 percent in 2004.

Underscoring the uncertainty, the Gallup Poll is using two versions of likely voters this year - a traditional one that asks about people’s past voting behavior and their current voting intentions and an expanded one that looks only at how intent they are on voting this year, which would tend to include more new voters.

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

Every poll has a margin of sampling error, usually about 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 United States. However - 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.

Not wildly, but that doesn’t make them less noticeable. There’s a big difference between a race that’s tied in the AP poll and one that gives Mr. Obama a 14-point lead in the Pew poll. Because of each poll’s margin of error, those differences may be a bit less - or more - than meet the eye.

That’s because each poll’s margin of sampling error should be applied to the support for each candidate, not the gap between them.

Take the AP poll, which has a margin of error of plus or minus 3.5 percentage points. Mr. Obama’s 44 percent support is likely between 48 percent and 40 percent. Mr. McCain’s 43 percent is probably between 47 percent and 39 percent.

When support for candidates is measured in ranges like that, some polls’ findings could overlap - or grow worse.

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



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