This paper provides a brief discussion and demonstration of regression toward the mean, a subtle statistical artifact that appears in the effort to measure change. Regression toward the mean frequently arises in educational assessment when repeated testing is used to determine achievement growth among students or schools at the extremes of the achievement distribution. This statistical artifact is important because it can lead to erroneous inferences about what is causing the observed changes in test scores. The demonstration makes use of Kentucky Core Content Test data.
Occasional Research Paper, No. 17, October 2010.