Wednesday, October 20, 2010

Nate Silver tries to disabuse political reporters of another favorite myth

In today's 538, Nate Silver takes down the myth that polls tend to show momentum. He left out some details that OE readers might be curious about (but, of course, we're not exactly the target audience), but it's an excellent piece of statistical writing.

The Misunderstanding of Momentum

Turn on the news or read through much of the analysis put out by some of our friends, and you’re likely to hear a lot of talk about “momentum”: the term is used about 60 times per day by major media outlets in conjunction with articles about polling.

When people say a particular candidate has momentum, what they are implying is that present trends are likely to perpetuate themselves into the future. Say, for instance, that a candidate trailed by 10 points in a poll three weeks ago — and now a new poll comes out showing the candidate down by just 5 points. It will frequently be said that this candidate “has the momentum”, “is gaining ground,” “is closing his deficit,” or something similar.

Each of these phrases are in the present tense. They create the impression that — if the candidate has gone from being 10 points down to 5 points down, then by next week, he’ll have closed his deficit further: perhaps he’ll even be ahead!

There’s just one problem with this. It has no particular tendency toward being true.

Read the rest...

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