Which statement about the relationship between observed score, true score, and error score is correct?

• A. error score = true score - observed score
• B. true score = observed score + error score
• C. observed score = true score - error score
• D. observed score = true score + error score

Answer: (D)

Observed score equals true score plus error score. This shows that what a student shows on a measure is their actual ability (the true score) combined with any measurement fluctuations (the error). If you know the observed score and the error, you can find the true score by subtracting the error, and if you know the true score and the error, you can find the observed score by adding them. For example, if the observed score is 85, the true score is 80, and the error is 5, then 85 = 80 + 5. The other formulas would flip signs or misplace adding versus subtracting, which would not align with how error contributes to the observed score.