August 20, 2008
Posted by Jay Livingston

Yesterday, Jeremy Freese had a post at Scatterplot on interaction effects:

There are so many ways of dividing a sample into subgroups, and there are so many variables in a typical dataset that have low correlation with an outcome, that it is inevitable that there will be all kinds of little pockets for high correlation for some subgroup just by chance.
Today, the Onion News Network had this example, probably inspired more by Mark Penn than by Jeremy, but instructive nevertheless.

Hat tip to Ezra Klein at The American Prospect.

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