Technically, the problem is that test scores (like Fahrenheit temperature, but unlike length, speed, or any number of other common measures) are not a ratio scale, which means that zero on the test score scale does not mean “zero achievement.” Zero on most scales is just an arbitrary point. Even on a raw-score scale, where zero means zero items answered correctly, it need not mean “no knowledge of the domain”; it just means no mastery of the particular material on the test. Percentage change is a meaningful metric only in the case of ratio scales.
