The last, and most common, way of measuring achievement is to use standardized test scores. Purely because they’re standardized, these tests are widely regarded as objective instruments for assessing children’s academic performance. But as I’ve argued elsewhere at some length, there is considerable reason to believe that standardized tests are a poor measure of intellectual proficiency. They are, however, excellent indicators of two things. The first is affluence: Up to 90 percent of the difference in scores among schools, communities, or even states can be accounted for, statistically speaking, without knowing anything about what happened inside the classrooms. All you need are some facts about the average income and education levels of the students’ parents. The second phenomenon that standardized tests measure is how skillful a particular group of students is at taking standardized tests – and, increasingly, how much class time has been given over to preparing them to do just that.