The Wild Differences in The Polls, Explained

By Andrew Satter, CQ Staff Fri Jun 27, 12:17 AM ET

Video: Pew Research Center editor Richard Morin explains discrepancies in different presidential polls

If you've been watching Presidential preference polls over the past week, you might feel a bit whipsawed.

For the second straight day, Gallup's daily tracking poll Thursday has John McCain and Barack Obama tied. Both candidates dropped a point from yesterday's tracking poll, down to 44 percent. The margin of error is +/- 2 percentage points.

The Rasmussen daily tracking poll in the same period of time has shown a 3-7 point gap between the two candidates.

Neither of the daily tracking polls square with two polls from that show Barack Obama holding a stunning double-digit lead. A Los Angeles Times/Bloomberg poll showed Obama with a 12 point lead over McCain, and a Newsweek poll had Obama leading by 15 points, if Ralph Nader and Bob Barr were included in the mix.

Why the difference? Were the Newsweek and L.A. Times biased in favor of Obama? Do the Rasmussen and Gallup pollsters favor McCain? Or maybe the public is wildly changing its views, daily.

All unlikely, says Richard Morin, a senior editor at the Pew Research Center. In an interview with CQ Politics, he said the discrepancy is probably a result of the Newsweek and L.A. Times/Bloomberg polls over-representing Democrats.

"When I look at those results, I know something is going on," said Morin.

"The first place that I look when I see these discrepancies, I look for the percentage of Republicans, Democrats and Independents in the sample. We know that the best predictor of how someone is going to vote is their party ID.

"Both the L.A. Times/Bloomberg and the Newsweek polls have (too) large percentage of Democrats and a (too) small percentage of Republicans."

While there are indeed more people who identify themselves as Democrats than Republicans in the country, Morin says the other polls, including Gallup, are more in line with the actual disparity than the Bloomberg or Newsweek polls.

"Interestingly enough," Morin said, "if you do the math and apply the proper percentages to the L.A. Times/Bloomberg and the Newsweek findings, you find that their results change dramatically."

In fact, Morin says, if the two polls that show Obama winning by a large margin were to modify their findings using the same percentage of Democrats and Republicans as other polls, Obama's lead would come down to somewhere between a toss-up and a small, single digit lead for Obama.

Morin cautioned that neither organization oversampled Democrats intentionally. It's a tricky business, getting exactly the right mix that mirrors the entire population.

"Embedded in the operations of polling are subtle differences that result in skews -- and they can be either Republican skews or Democratic skews."

Andrew Romano writes in his Newsweek blog, 'Stumper:' "The problem here is that unlike race, age and gender, party ID is fluid--and even extreme swings might reflect actual changes in the mood of the electorate. 'This is a canard,' says NEWSWEEK polling maestro Larry Hugick when asked about the McCain memo [which charged that the party ratios were wrong]. 'Both parties do it. But ID isn't a fixed property. In fact, it's associated with the candidates. It's been proven that as a candidate goes up in the polls, so does his party. Same when a candidate goes down.'"

As the season wears on, there are likely to be plenty of differences among the slew of surveys. Pollsters commonly caution that each poll is "just a snapshot in time." And the focus has to be just right.