Friday, March 21, 2003

Sample Selection Bias

Another poll used inappropriately via the Volokh Conspiracy.



[Eugene Volokh, 5:26 PM]
REPEAT AFTER ME: "I will not believe scientifically invalid polls." "I will not believe scientifically invalid polls." "I will not believe scientifically invalid polls, even if I like their results."

Look, I'd like to think that most MTV viewers support the war; in fact, they might well support the war. But unless I misunderstand the way the poll described here (see also the InstaPundit link) was conducted, that poll doesn't tell us that. It doesn't tell us much of anything, because it counts only those people who choose to vote in it (key phrase: "Among people voting in MTVNews.com's polls . . .," and see also this sample of an mtv.com poll), and we have no reason to believe that they're a representative sample of MTV viewers, or of any other group.

UPDATE: Drat, The Corner falls for this, too.



Remember, if you want to make accurate inference about a population, your sample selection methods need to be free from systematic biases...this example clearly fails that test. Although I doubt they were trying to make accurate inference...

Keywords: Polls, BUS230

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