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Polls' Accuracy No Simple Yes Or No

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The Associated Press

The Tampa Tribune

Published: October 24, 2008

WASHINGTON - Barack 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.

Confused? 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 leading by 11 percentage points 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 about why the polls differ.

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 from others. And while most polling organizations, including the AP, do not modify the responses to reflect some recent tally of how many Democrats, Republicans and independents there are, some do.

Are those the only changes made?

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.

Is that hard to do?

Quite hard, because no one will truly know who will vote on Election Day until that day is over.

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 voters and minorities. The question pollsters face is whether, and how, to adjust their tests for likely voters to reflect this.

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