Using value-added and other types of growth model estimates in teacher evaluations is probably the most controversial and oft-discussed issue in education policy over the past few years.

Many people (including a large proportion of teachers) are opposed to using student test scores in their evaluations, as they feel that the measures are not valid or reliable, and that they will incentivize perverse behavior, such as cheating or competition between teachers. Advocates, on the other hand, argue that student performance is a vital part of teachers’ performance evaluations, and that the growth model estimates, while imperfect, represent the best available option.

I am sympathetic to both views. In fact, in my opinion, there are only two unsupportable positions in this debate: Certainty that using these measures in evaluations will work; and certainty that it won’t. Unfortunately, that’s often how the debate has proceeded – two deeply-entrenched sides convinced of their absolutist positions, and resolved that any nuance in or compromise of their views will only preclude the success of their efforts. You’re with them or against them. The problem is that it’s the nuance – the details – that determine policy effects.

Let’s be clear about something: I’m not aware of a shred of evidence – not a shred – that the use of growth model estimates in teacher evaluations improves performance of either teachers or students.

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