Almost everyone agrees that teachers are the key factor in whether a child learns at school. While a teacher cannot control who enters her classroom—children arrive with a wide variety of attainments and family resources—she can have a powerful effect on the gains a child makes while in her classroom. Scientific measurement of the gains that a teacher produces is the essence of value-added calculations, described in the earlier chapter on the subject. The beauty of value-added calculations is that they permit a state like Arkansas to reward teachers directly for raising pupil achievement. The teacher reward schemes discussed in this section are founded on value-added calculations. Their key virtue is that they can provide the vast majority of teachers in a state with consistent, powerful incentives to raise achievement. Yet, the schemes are also flexible: they can be designed to include schoolwide incentives, instructional team incentives (an instructional team is, for instance, all fourth grade teachers in a school), incentives to teach in schools with particularly low-achieving children, and incentives to teach classes that are particularly hard to staff.