From "joejoejoe," regarding yesterday's release of a new national survey from CNN/ORC:
Here's the headline to the CNN story that accompanies the poll.
'CNN poll: Obama, McCain in a statistical dead heat'
I'm not sure why a 5-point lead in a poll with a margin of error of +/- 3.5% is a "statistical dead heat" but whatever. Based on the '04 turnout a 1.5% victory projects to about 1.8 million more votes for Obama then for McCain. Doesn't "dead heat" mean tie?
Yes, it does, as as such "statistical dead heat" is a phrase we wish journalists would avoid.
To be fair, Nate Silver made the same point more emphatically (citing a National Council of Public Polls release) a few hours before joejoejoe. But we appreciate our alert readers nonetheless.
Update: A highly valued reader emails:
Not exactly. Suppose there are no undecideds and Obama leads 53-47. The +/- 3.5% MOE means that the estimate of 53% for the Obama has a 95% confidence interval ranging from 49.5% to 56.5%. 49.5% for Obama means 50.5% for McCain, so a McCain lead is within the margin of error. It's a little more complicated when there are undecideds, but the result would be similar
I probably posted this item too quickly. To me, and to most readers, "dead heat" means "tie." The point I agree with -- and the one made more directly by Nate Silver -- was not to imply that the 5 point margin was outside the margin of error, but rather to object to the use of the phrase "statistical tie" to describe a difference that is not quite large enough to attain statistical significance. It presumes we know the race is "a tie" when we lack the evidence, from this one poll, to be certain that a candidate is ahead.
Caution is always in order when it comes to interpreting small differences on just one poll result, but we have more than one poll to consider. Since May 1, we have logged 37 national poll releases (omitting daily tracking releases based on over-lapping samples). Only one (from Gallup) showed a "tie" result (44% to 44%). The other 36 had Obama ahead by margins of 1 to 15 percent. That's evidence that "tie" is not the best way to describe the current preferences in the race for president.