Regression to the Mean
Regression to the mean describes the tendency for extreme pretest scores to become less extreme on a posttest. Regression to the mean is a threat to internal validity in repeated measures experimental design.
Smith, R. A. (2005). Regression to the mean. In B. S. Everitt, & D. C. Howell (Eds.), Encyclopedia of statistics in behavioral science (1st ed., ). Hoboken, NJ: John Wiley & Sons, Ltd. doi:10.1002/0470013192.bsa553