Education agencies are evaluating teachers using student achievement data. However, very little is known about the comparability of test-based or "value-added" metrics across districts and the extent to which they capture variability in classroom practices. Drawing on data from four urban districts, we found that teachers were…

High-stakes tests are the most heavily weighted measures in accountability systems developed in response to No Child Left Behind. While some studies show high-stakes accountability being related to test score gains, others suggest these policies do not improve achievement and often result in unintended consequences. To understand mechanisms…