What term describes the phenomenon where extreme scores tend to move closer to the average upon retesting?

The term that describes the phenomenon where extreme scores tend to move closer to the average upon retesting is statistical regression to the mean. This concept is grounded in the principle of probability and statistics, indicating that when a variable is extreme on its first measurement, it is likely to be closer to the average on subsequent measurements due to random error, natural variation, or the inherent unpredictability of the measurement process.

For instance, if a participant scores very high or very low on an initial test, factors influencing their performance, such as luck or temporary conditions, may not be as present during the retest. As a result, scores typically gravitate towards the mean, reflecting a more accurate assessment of ability or performance over time. This understanding is crucial in research design, particularly when interpreting the results of assessments and understanding changes over time in a population.

The other terms provided do not accurately describe this phenomenon. Testing effects pertain to how prior exposure to a test can influence subsequent performances, which is distinguishable from regression to the mean. Experimental mortality refers to the loss of participants in a study, affecting the validity of the results, while within-group differences highlight variability among subjects within the same group but does not specifically address how extreme scores behave over time.