LA Times nonsense on post-debate poll

The only thing worse than newspapers failing to report on other polls is the way they (mis)interpret their own findings.

The Los Angeles Times is currently running a story titled "Obama slightly widens lead after debate, poll finds" (via Yglesias). The basis for this claim is a 1% increase in Obama's support and a 2% increase in Obama's lead among voters in their poll who watched the debate (presumably a non-random group):

The much-anticipated first presidential debate between John McCain and Barack Obama on Friday appears to have helped Obama slightly widen a lead over his Republican opponent, a post-debate Los Angeles Times/Bloomberg survey shows.

Registered voters who watched the debate preferred Obama, 49% to 44%, according to the poll taken over three days after the showdown in Oxford, Miss.

That is a small gain from a week ago, when a survey of the same voters showed the Democratic candidate with a 48% to 45% edge.

An increase of 1% in Obama's poll numbers (or a 2% increase in his lead among the group that watched the debate) is meaningless. There are two problems. First, we can't be sure that the increase among the group who watched the debate is meaningful. Any time you survey people twice on their vote preferences, there may be minor fluctuations. A difference of 1-2% is almost surely not statistically significant. Second, we can't extrapolate anything meaningful from that group's change to the larger population of registered voters as a whole. The reason is that the voters who didn't watch the debate may also change their preferences as a result of media coverage, external events, etc.

Who allowed this nonsense to be published in a major newspaper? It's the statistical equivalent of 2+2=5.

Update 9/29 8:41 AM: The description of the problems with the article above has been improved and corrected with help from Joel Wiles.

Update 9/29 9:10 PM: Per Jonny Drake's comment below, the wording of the post has been clarified to distinguish between a 1% increase in Obama's support and a 2% increase in his lead among voters in the Times poll who watched the debate.

Comments

[Jonny Drake]

If Obama went from a 48%-to-45% lead (ahead by three points) to a 49%-to-44% lead (ahead by five points), he gained two points, not one.

[Sean-B]

So does Matthew Yglesias think the first debate is significant? He seems to. So does James Fallows. And, like the other commenter mentioned, it's 2%, not 1%, which some national polls are mirroring. I don't understand the mystery: what else happened between Friday and Sunday besides the debate to change those people's minds? Did McCain's limo run over someone's dog?

When the Times says "it appears to have helped slightly", based on a poll given within a small time frame asking how people how they felt "after the debate", the odds are, seeing as that's the only significant political event that happened between Friday and Sunday and 100% of respondents said they saw it, it was the debate or discussion of it that shaped their view in that very short period of time.

The upcoming vice presidential debate, depending on how each candidate performs according to the narrative that their campaign, their opponent's campaign or the media has created for them, will surely affect polls that follow shortly after, even if it's "only" 1%. I haven't found one respected writer on the internets who doesn't think that this debate is critical to McCain's campaign.

Will your bias against debates really stop you from watching it, Brendan? Because I don't know a political junkie of any stripe that will be able to keep away. Hope you find something to blog about on Friday!