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July 02, 2008

Margin of error

Stuff like this really grates my nerves.


With the dust having finally settled after the prolonged Democratic presidential primary, a new CNN/Opinion Research Corporation poll shows Sens. John McCain and Barack Obama locked in a statistical dead heat in the race for the White House.

With just over four months remaining until voters weigh in at the polls, the new survey out Tuesday indicates Obama holds a narrow 5-point advantage among registered voters nationwide over the Arizona senator, 50 percent to 45 percent. That represents little change from a similar poll one month ago, when the presumptive Democratic presidential nominee held a 46-43 percent edge over McCain.

CNN Polling Director Keating Holland notes Tuesday's survey confirms what a string of national polls released this month have shown: Obama holds a slight advantage over McCain, though not a big enough one to constitute a statistical lead.

"Every standard telephone poll taken in June has shown Obama ahead of McCain, with nearly all of them showing Obama's margin somewhere between three and six points," Holland said. "In most of them, that margin is not enough to give him a lead in a statistical sense, but it appears that June has been a good month for Obama."

[...]

The poll, conducted June 26-29, surveyed 906 registered voters and carries a margin of error of plus or minus 3.5 percentage points.



I realize the media loves a horse race, but let's put this into perspective. I grant that a five-point lead in a poll with a 3.5-point MOE is not statistically significant, but calling it a "dead heat" is a huge overbid. What "margin of error" means is that the "true" level of support for each candidate is a range, from the reported number minus the MOE to the reported number plus the MOE, and that this will be an accurate statement 95% of the time. Putting it another way, the CNN poll means Obama's "true" level is support is between 46.5 and 53.5%, while McCain's is between 41.5 and 48.5%.

The "dead heat" or "statistical tie" assertions come from the observation that the low end for Obama is less than the high end for McCain. However, and the reason why I object to those terms being used so freely, the odds of that being the case are pretty slim. The probability that Obama holds an actual lead over McCain is almost 94%, going by the calculations provided here; use the enclosed Excel spreadsheet and plug in the appropriate numbers to see for yourself. That give a very different perspective than "dead heat" and "statistical tie", doesn't it?

One more thing:


But the new CNN/ORC polls shows the race gets even tighter when the two most prominent third-party presidential candidates are considered. In a four way match-up that includes independent candidate Ralph Nader and Libertarian candidate Bob Barr, Obama's lead over McCain dwindles to 3 percentage points, 46 percent to 43 percent. (Nader registers 6 percent while Barr gets 3 percent.)

But it remains unclear just how much effect Nader and Barr will have on the election, as summertime surveys often overstate the eventual Election Day showing of third-party candidates.

"A useful rule of thumb is that third-party candidates in November get no more than half the support polls show them having in June or July," Holland said.



Write this down: No way does Ralph Nader get more than one percent of the vote in November. Throw out any poll result that suggests otherwise. He's got no money and will get a tiny fraction of the free media he received in 2000, when he collected 2.7% of the vote. He's a non-factor and should not be treated otherwise.

UPDATE: Nate Silver offers some evidence of the media's desire to make this race look closer than it now is.

Posted by Charles Kuffner at July 2, 2008 05:43 AM

Comments

Polls are meaningless. Are these the same polls that had both Kerry and Gore winning the last 2 elections?

Posted by: Royal at July 2, 2008 02:05 PM

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