United States Government Accountability Office
This short GAO report summarizes and explains "growth models," which are methods for evaluating the change in performance of students or schools over time and are often referred to as "academic value added." The report's growth model definition is broad (some methods included here don't necessarily gauge whether particular students are improving but rather evaluate whether a school has higher-achieving third-graders this year than last), so it's not surprising that 26 states were found to be using some sort of growth model at the time of publication and 22 more were developing such. And NCLB is making use of growth models, too. For example, schools that miss proficiency targets but still reduce their numbers of "not proficient" students by at least 10 percent are sheltered under NCLB's safe harbor provision--a growth model according to the GAO's definition. And California uses its Academic Performance Index, another growth model, as the "additional indicator" its schools must meet in order to make adequate yearly progress. Still, North Carolina and Tennessee are the only two states whose robust growth models met all criteria for approval under a Department of Education pilot program--thus, those states' growth models can be used to calculate adequate yearly progress. But growth models are good for more than NCLB reporting. In North Carolina, for example, the state uses its to determine teacher bonuses. This is a fairly useful report which includes a timeline of when states began using their particular models and brief explanations of how the various models work. It won't begin to help one understand the nuances of something as complex as Tennessee's value-added model, but it does provide a useful overview of the landscape. You can find it online here.