Economists find value-added measures (roughly, the change in student test scores from one year to the next) enormously useful in measuring educational outcomes. Whether VAM is so useful for teacher personnel policy is a whole lot less clear. In an article aimed at policy makers, Sean Corcoran and Dan Goldhaber summarize what we know and then engage in a friendly dispute over the usefulness of VAM for evaluating teachers.
A few excerpts:
…the potential for VAM measures to dramatically improve teaching effectiveness and the quality of the profession tends to be overblown. Student achievement should be an important part of a new and improved system of evaluation…. however, VAM measures lack transparency and are inherently limited by imprecision.
VAM measures may turn out to be useful [for] …separating the very high- and very low-performing teachers from the rest of the pack. … Their utility as a job performance indicator for a significant number of teachers is another matter…
I would argue that VAMs ought to be used for three primary reasons. First, evaluation is to some extent about determining which teachers should stay in the profession … some, perhaps quite small, proportion of teachers are not very effective and ought to be dismissed.
Second, I see value added as a catalyst for broader changes to teacher evaluation. I am quite skeptical that we would be engaged in what is now almost a national experiment with new, and hopefully more rigorous, teacher evaluation were it not for the specter of VAM usage.
Third, and perhaps most importantly, evidence suggests that VAM measures are better at predicting future student test achievement than the other credentials—licensure, degree, and experience levels—that are now used for high stakes purposes…To the extent that evaluations are in fact used for any purpose, we would want them to be good predictors of student achievement and, although imperfect, VAMs look pretty good compared to the other options currently out there. There is a laser focus on the known flaws of VAMs while other methods of teacher evaluation have basically been given a pass.
Note the nuanced views. Neither Corcoran nor Goldhaber think the use of VAM scores for teacher evaluation either a great evil or a panacea.