This is measured by variance and standard deviation. Variance is a useful number because it is additive. For instance the total variance in a group of scores could be the sum of the valid variance, random variance, and bias variance. Standard deviation is useful because it is in the same units as the tests. Skew Some distributions are skewed in the positive or negative direction relative to a perfect bell curve.
T scores are easier to deal with than Z scores.
E.g., Billy has the vocabulary of a twelve year old. Grade-equivalent and developmental scales are similar to age scales but use grade scales or developmental scales rather than ages as the basis for comparison. E.g., You are at a ninth grade reading level. The strength is that they are very intuitive. They tend to overemphasize the importance of differences around the mean and underestimate the importance of differences at the extreme.