If the observed score is 90 and the true score is 88, what is the error score?

• A. 0
• B. -2
• C. 1
• D. 2

Answer: (D)

The idea is that what we observe on a test equals the true score plus any measurement error. So to find the error, subtract the true score from the observed score: 90 minus 88 equals 2. This means the observed score is higher than the true score by 2 points, indicating a positive error. If you plug in the other values, they wouldn’t produce the observed score when starting from the true score: 0 would give 88, -2 would give 86, and 1 would give 89. Only 2 matches 90 = 88 + 2.