It’s worth your while to read this article in a paper from New York, concerning a lot of the problems attendent and inherent in the so-called Value Added Measurements.
A few quotes from the article:
“[…] 31 percent of English teachers who ranked in the bottom quintile of teachers in 2007 had jumped to one of the top two quintile by 2008. About 23 percent of math teachers made the same jump.
“There was an overall correlation between how a teacher scored from one year to the next, and for some teachers, the measurement was more stable. Of the math teachers who ranked in the top quintile in 2007, 40 percent retained that crown in 2008.
“The weaknesses of value-added detailed in the report include:
“the fact that value-added scores are inherently relative, grading teachers on a curve — and thereby rendering the goal of having only high value-added teachers ‘a technical impossibility,’ as Corcoran writes
“the interference of imperfect state tests, which, when swapped with other assessments, can make a teacher who had looked stellar suddenly look subpar
“and the challenge of truly eliminating the influence of everything else that happens in a school and a classroom from that ‘unique contribution’ by the teacher