What is the most significant source of error in a test-retest design?

• A. Random measurement error.
• B. Change in subject situation over time.
• C. Instrument calibration drift.
• D. Testing environment differences.

Answer: (B)

In a test-retest design, the key idea is that the trait being measured is stable between the two assessments. The most significant threat to this stability is a change in the subject’s situation over time—things like mood, fatigue, motivation, practice effects, or life events—that can shift the true score. When the true score changes, the differences in observed scores reflect real change rather than measurement noise, which lowers the reliability of the test.

Other sources, such as random measurement error, instrument drift, or differences in testing environment, can affect scores as well, but they typically cause less impact than genuine changes in the subject’s condition between administrations.