If the mean observed score in a distribution of repeated tests is the best estimate of the true score, it should make sense that all the other scores in the distribution reflect, or contain, some amount of error. So, observed scores that are + 1 SD from the mean of this distribution are better reflections of true scores than are scores + 2 SDs from the mean. Scores that are + 3 SDs from the mean reflect even more error. 10. Classical test theory uses the standard deviation of this distribution as a basic measure of error called the which tells us, on average, how much an observed score deviates from the true score