Adventures in Polling

How do you explain the variations in these polls, all issued today:

AP presidential poll: All even in the homestretch

WASHINGTON (AP) - The presidential race tightened after the final debate, with John McCain gaining among whites and people earning less than $50,000, according to an Associated Press-GfK poll that shows McCain and Barack Obama essentially running even among likely voters in the election homestretch.

The poll, which found Obama at 44 percent and McCain at 43 percent, supports what some Republicans and Democrats privately have said in recent days: that the race narrowed after the third debate as GOP-leaning voters drifted home to their party and McCain's "Joe the plumber" analogy struck a chord.

Compare that to this one:

Reuters/C-SPAN/Zogby Poll: And Then It Was Ten . . .
Obama 51.6%, McCain 42.0%

And this one:

Battleground Poll: Obama 49%, McCain 47%

And this one:

FOX News Poll: Obama Grabs Nine Point Lead Over McCain
Wednesday, October 22, 2008

Less than two weeks before Election Day, Barack Obama leads John McCain 49 percent to 40 percent among likely voters, according to a FOX News poll released Wednesday.

And this one:

IBD/TIPP Poll: Obama 45.7%, McCain 42%

They can't all be right. Michael Barone asks the obvious question: Are the Polls Accurate?