Adequate Yearly Progress

What is "adequate yearly progress"? How does measuring it help to improve schools?

No Child Left Behind requires each state to define adequate yearly progress for school districts and schools, within the parameters set by Title I. In defining adequate yearly progress, each state sets the minimum levels of improvement--measurable in terms of student performance--that school districts and schools must achieve within time frames specified in the law. In general, it works like this: Each state begins by setting a "starting point" that is based on the performance of its lowest-achieving demographic group or of the lowest-achieving schools in the state, whichever is higher. The state then sets the bar--or level of student achievement--that a school must attain after two years in order to continue to show adequate yearly progress. Subsequent thresholds must be raised at least once every three years, until, at the end of 12 years, all students in the state are achieving at the proficient level on state assessments in reading/language arts and math.