How Useful Are Grade Distributions?
If the groups of AP students performed identically from year to year in their achievement of the AP course objectives, these grade distributions would be the only information necessary for maintaining standards. Choosing the grade boundaries would be a simple task of finding the boundaries that would create the same percentage of grades 5, 4, 3, 2, and 1 as the previous year's exam. However, each year there are new schools participating in the AP Program, and new teachers at schools that participated the previous year. Each year, the teachers who previously participated have become more experienced. Therefore, the achievement of the student population taking an AP Exam cannot be assumed to remain constant from year to year.

It is reasonable to assume, though, that in most cases there will be no extreme changes in the students' general level of achievement, particularly for exams that are not new, and for which the number of students has not changed greatly. So grade distributions are a useful tool, when looked at along with other forms of evidence.