Are polls accurate in presidential race?

By Andy Sullivan

WASHINGTON (Reuters) - With the U.S. presidential election less than a week away, Democrat Barack Obama holds a steady lead over rival Republican John McCain in opinion polls, leading many pundits to say McCain is effectively finished.

Could the polls be wrong?

They have misled before. The most famous time came in 1948 when polls showed Republican Thomas Dewey on his way to winning the White House but missed the late surge that carried Democrat Harry Truman to victory.

More recently, polls showed Obama ahead of Democratic rival Hillary Clinton in the January New Hampshire primary by an average of 8 percentage points. Clinton won.

McCain's campaign thinks it could happen again on November 4.

"All signs say we are headed to an election that may easily be too close to call by next Tuesday," McCain pollster Bill McInturff wrote in a memo released on Tuesday.

Pollsters are careful to say their work does not predict a race's outcome but only captures a snapshot of the electorate at a certain point in time.

And there is always the possibility of error in a discipline that combines science with a certain amount of guesswork.

"We are engaged to some degree in some artwork and assumption," said pollster John Zogby, whose Reuters/C-SPAN/Zogby tracking poll shows Obama leading by 7 percentage points.

Pollsters can't simply tally up the results of their telephone surveys but must make educated guesses about who will actually show up to vote.

These "likely voter" models vary from poll to poll, leading to results that can vary as well.

Rasmussen Reports' daily tracking poll on Wednesday showed Obama leading McCain by 3 percentage points, while a survey released on Tuesday by Pew Research Center for the People and the Press showed Obama leading McCain by 15 percentage points.

HIGHER TURNOUT AMONG BLACKS?

Gallup publishes two separate polls based on varying likely-voter models. Its traditional model showed Obama ahead by 2 percentage points on Wednesday, while its "expanded" model, which assumes higher turnout rates among minorities and young people, showed Obama leading by 7 percentage points.

Zogby's poll and the Pew poll also assume that black voters, inspired by the chance to elect Obama as the first black president, will make up a higher percentage of the electorate this year.