The system calculates the value teachers add to their students’ achievement, based on changes in test scores from year to year and how the students perform compared with others in their grade.

Michelle A. Rhee, the schools chancellor in Washington, fired about 25 teachers this summer after they rated poorly in evaluations based in part on a value-added analysis of scores

heir use spread after the 2002 No Child Left Behind law required states to test in third to eighth grades every year, giving school districts mountains of test data that are the raw material for value-added analysis