If no one knew that you were going to evaluate schools by looking at test scores of students, test scores might provide a reasonable way of measuring schools’ effectiveness. But once teachers and administrators recognize that test scores will be used to evaluate their effectiveness, they have every incentive to find ways of raising their students’ scores—even at the expense of the quality of their education. They may “teach to the test,” for example, rather than teaching critical thinking. The consequences are twofold. Once this happens, test scores lose much of their value as a means of assessing a school’s performance. The process of measurement may even actively undermine the quality of education, as time and effort are shifted away from valuable activity and toward the kind of memorization that improves test scores but is nearly useless from an educational perspective.