Which equation expresses the relationship between observed score, true score, and error score?

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

Answer: (C)

Observed score is the result of adding the true score to the measurement error. In testing, what you aim to measure is the true score, the part reflecting actual ability or trait, but the score you see on the test also includes random fluctuation from the measurement process—this is the error. So the direct, most informative way to express their relationship is observed score = true score + error. It shows clearly that the observed value comes from combining the true value with some error term.

The other forms are just rearrangements of the same idea (solving for error or solving for true). They convey the same relationship numerically, but the forward form emphasizes how the observed score is formed from true score plus error.