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California State University, Dominguez Hills
University of Wisconsin, Parkside
Created: April 16, 2004
Latest Update: April 16, 2004
jeannecurran@habermas.org
takata@uwp.edu

Regression Towards the Mean

Site Copyright: Jeanne Curran and Susan R. Takata and Individual Authors, April 2004.
"Fair use" encouraged.

"Statistical regression. Journalists have been easy prey to this one. A school board announces a program to focus on its worst-performing schools and improve them. It picks the two or three schools with the worst test scores in the previous year and lavishes attention and new teaching methods on them. Sure enough, the next year those schools have better test scores. The problem is that they would have done better even if there had been no special attention or new technique.

"The reason is that there is a certain amount of random error in all tests and rankings. The schools at the bottom of the list got there partly by chance. Give them a new roll of the dice with next year's testing, and chance alone will move them closer to average. The phenomenon is call regression toward the mean, because it always moves the extreme performers, top and bottom, closer to the mean on the second test. It is a danger any time that you select the extremes of a distribution for treatment. Most educators know about it, but knowing about it doesn't stop them from taking the credit for it."

From Regression Towards the Mean by Philip Meyer

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