According to the latest poll ... as election day nears, it's one poll after another. But what do they really mean?

• Author: Smith, Patricia

As soon as the Republican Convention wrapped up at the end of August, pollsters scrambled to measure its effect on voters.

Rasmussen, one of the major polling firms, released a poll showing 48 percent of voters favoring Mitt Romney, the Republican presidential candidate, and just 44 percent for President Obama, the Democrat.

Good news for Romney, right? Not so fast. A Gallup poll showed Obama leading by one point. A third poll, by Ipsos/ Reuters, showed them in a dead heat at 45 percent each.

So what's going on? Why are three different polls taken the same week coming up with such different results? The short answer: Polls are complicated.

Politicians use polls to find out what voters think and care about and to tailor their messages to them. The news media use polls to give the public a sense of which candidate is ahead at any given time and why. Polls can have both direct and indirect effects on voters--especially in a tight contest like this year's race for the White House.

"They can stimulate people to give money or to volunteer for a campaign," says Michael Traugott, a polling expert at the University of Michigan. "They can make people think that their vote is worth more if the margin is close, meaning they have an impact on turnout."

It's just as important, however, to know the limits of any individual poll.

First, a poll tells us about the present, not the future: It's not a crystal ball, but a snapshot of public sentiment at a particular moment--and not a perfectly sharp snapshot.

The basic premise behind polling is that by questioning a surprisingly small number of people, you can get a good sense of what an entire city or state or country is thinking.

Most reputable political polling relies on a method called "probability sampling": If you select people at random from a population, you have a good chance of reflecting, within a few percentage points, the opinions of everyone.

Swing Voters

But there are many possible sources of distortion. How questions are worded--and even the order in which they're asked--can sway the results.

And then there's the electorate itself, which isn't always easy to pin down. Although a large portion of the American public is committed to one party or the other, there's always a segment that wavers, or stays undecided (or uninterested) until late in a campaign. Those swing voters often determine who wins, and their views can keep changing up to the very last minute.

One of the most embarrassing errors...