Diane Ravitch writes in The New York Review of Books: Many researchers and testing experts have cautioned that evaluating teachers by the test scores of their students—called value-added assessment—is fraught with problems. Linda Darling-Hammond, a prominent scholar at Stanford University and one of the nation’s leading authorities on issues of teacher quality, has written that the measures say more about which students are in the classroom than about the competence of the teacher. The National Academy of Education and the American Educational Research Association issued a joint statement saying the same thing. Those who teach students with disabilities, English-language learners, and low-performing students are likely to get smaller gains in test scores than those who teach students from affluent homes in well-funded schools. Using test scores to rate teachers will penalize those who teach the students in greatest need. Over time, teachers will avoid the students who jeopardize their jobs and their reputations. This will be harmful to the students who need talented and experienced teachers most urgently. Across the nation, as districts put into effect the “reform” that Secretary Duncan wants, the consequences have been counterproductive.