Given that the properties of value -
added measures differ across grades and subjects, policymakers should consider using different methods for calculating and using value - added in different grades and subjects.
In addition to the fact that the tests are narrow and do not
measure higher - order thinking skills, researchers have found that value -
added models of teacher effectiveness are highly unstable: Teachers» ratings
differ substantially from class to class and from year to year, as well as from one test to the next.