AP presidential poll: All even in the homestretch

AP presidential poll: All even in the homestretch: "Obama and McCain were essentially tied among likely voters in the latest George Washington University Battleground Poll, conducted by Republican strategist Ed Goeas and Democratic pollster Celinda Lake. In other surveys focusing on likely voters, a Washington Post-ABC News poll showed Obama up by 9 percentage points, while a poll by the nonpartisan Pew Research Center had Obama leading by 14. A Wall Street Journal/NBC News poll, among the broader category of people registered to vote, found Obama ahead by 10 points.

Polls are snapshots of highly fluid campaigns. In this case, there is a margin of error of plus or minus 3.5 percentage points; that means Obama could be ahead by as many as 8 points or down by as many as 6. There are many reasons why polls differ, including methods of estimating likely voters and the wording of questions.

Charles Franklin, a University of Wisconsin political science professor and polling authority, said variation between polls occurs, in part, because pollsters interview random samples of people.

'If they all agree, somebody would be doing something terribly wrong,' he said of polls. But he also said that surveys generally fall within a few points of each other, adding, 'When you get much beyond that, there's something to explain. "