The Way Presidential Polls Work - Or Don't

The only thing predictable about presidential polls lately is how unpredictable they are. As the election draws near, new polls come out almost daily. But they often have very different results from the one that came before. How does this happen? CBS News senior political correspondent Jeff Greenfield runs the numbers.

You've heard this all before: Great news for Barack Obama! He's 13 points up. No, wait, great news for John McCain, he's only one point behind, or is it 3 ... or 7 ... or 10?

Well it's all of those. The spread between the candidates depends on which poll you're looking at. And it's the same with key states.

Obama's up either 2 points in Virginia or he's up 10 points in Virginia. McCain is up 1 point in Ohio or 12 points down in the Buckeye State.

How can that race vary this widely? Because of the way polls work - or don't work.

First, different polls make very different assumptions about who's likely to vote.

"If you assume that you have to have voted before, you probably undercount your voters," said CBS News polling director Kathy Frankovic. "If you trust them and when they tell you they're definitely going to vote, you may be representing them accurately. You may possibly be overestimating their share of the electorate."

Then, there's the cell-phone vote. Today, almost 12 percent of Americans don't have land line phones at all. Some polls - like ones conducted by CBS News - can reach them. But polls that use automatic dialing techniques can't.

"The cell phone-only people are disproportionately young, but not all of them are young. There still are a significant number of middle-aged folks who are only reachable on their cell phones," Frankovic said.

In fact one study shows polls that include "cell phone-only" folks give Obama 3 points more than polls that don't include such voters.

Another point: When a poll says it has a margin of error, you have to understand what it means. It means it could be off by 3 points in either direction for either candidate.

So, if Obama has a 50 to 45 lead, it could mean anything from a 53 - 42 edge, to a 48 - 47 edge for McCain.

Finally, some polls can be just plain wrong. And people can change their minds.

This is why Kevin Nealon on SNL years ago reported that "in a recent poll, 85 percent said if the election were held today - they'd really be surprised."