Regression to the mean
The 20 lowest scorers on a computer-scored test are given tutoring. On a retest a month later, their average score rises. With no control group, which explanation must be ruled out before crediting the tutoring?
Answer D: explanation for every choice
A
A ceiling effect limits gains among high scorers. These students started at the bottom.
B
Social desirability bias affects self-report measures, not test scores.
C
Computer scoring removes any chance for the observer's expectations to affect the scores.
D
Correct. Extreme scores partly reflect chance, so a group selected for very low scores will tend to score closer to average on retest even without help.
TakeawayGroups selected for extreme scores drift toward the mean on retest.
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Reviewed 2026-11-05 by a medical student. Spot an error? Report it with question ID ps155.