Which statement best describes the error score?

• A. The observed score a student actually receives.
• B. The score that would be obtained if measurement error were zero.
• C. The difference between observed score and true score.
• D. The sum of true score and observed score.

Answer: (B)

Think of measurement error as the part of a test score that comes from noise, inconsistencies, or randomness in the measurement process. In classical test theory, the observed score is what a student actually earns, and it can be thought of as the true score plus an error component: X = T + E. The error score is this E, the amount the observed score deviates from the true score. Put another way, the error score is the difference between what is observed and what would be obtained if measurement error were zero (the true score). So the correct way to describe the error score is as the difference between the observed score and the true score. The score with no error is the true score, not the error score, and the other options either describe the observed score, or incorrectly combine true and observed scores.