Measuring schools’ and teachers’ contributions to student achievement is a central part of school accountability systems. The key idea behind accountability systems is providing information and incentives so that the people who staff schools make better decisions and thereby improve student learning. If we can measure teachers’ and schools’ contributions accurately, we have the information we need to diagnose learning problems, management problems, teachers’ skill deficits, and so on. Because measuring teachers’ and schools’ contributions to achievement is central to an accountability system, using accurate measures is key. In particular, it is important that teachers and schools be held accountable for their contributions to achievement, not outcomes outside of their control. Only part of a student’s current achievement is, however, determined by his current teacher and school. His level of achievement also reflects what he learned from past teachers and schools, from his family, and from his peers.