Measurement in education presents many problems, including what is to be measured, and the accuracy of measurement. Understanding statistical distributions and what a measurement means, given the distribution, is very important. Special education requires thinking about how distributions can and should be changed as well as the effects of instruction on individual learners. Exceptional learners may be compared with themselves, but it is ultimately important to compare their performance with that of others, including typical learners. However, judging special education's effectiveness by comparing the achievement of students with disabilities with that of those without disabilities is unfair and irrational. Most people would like to see average educational performance improve. This is not just possible but probable if most students are taught better. But many people seem to want to decrease the discrepancies between high and low scorers to shrink—the variance so that students become more like one another.