Regression Toward the Mean (4 of 6)
Regression toward the mean will occur if one chooses the lowest-scoring subjects
in an experiment. Since the lowest-scoring subjects can be expected to have been
unlucky and therefore have scored lower than their "true" scores, they
will, on average, improve on a retest. This can easily mislead the unwary researcher.
What if a researcher chose the first-grade children in a school system that scored
the worst on a reading test, administered a drug that was supposed to improve
reading, and retested the children on a parallel form of the reading test. Because
of regression toward the mean, the mean reading score on the retest would
almost certainly be higher than the mean score on the first test. The
researcher would be mistaken to claim that the drug was responsible for
the improvement since it would be expected to occur simply on the basis
of regression toward the mean.
Consider an acutal study that received considerable media attention. This study
sought to determine whether a drug that reduces anxiety could raise SAT
scores by reducing test anxiety. A group of students whose SAT scores were
surprisingly low (given their grades) was chosen to be in the experiment.