I often hear the following argument: Improving teacher quality is more cost-effective than other options, such as reducing class size (see here, for example). I am all for evaluating policy alternatives based on their costs relative to their benefits, even though we tend to define the benefits side of the equation very narrowly – in terms of test score gains.
But “improving teacher quality” cannot yet be included in a concrete costs/benefits comparison with class size or anything else. It is not an actual policy. At best, it is a category of policy options, all of which are focused on recruitment, preparation, retention, improvement, and dismissal of teachers. When people invoke it, they are presumably referring to the fact that teachers vary widely in their test-based effectiveness. Yes, teachers matter, but altering the quality distribution is whole different ballgame from measuring it overall. It’s actually a whole different sport.
I think it is reasonable to speculate that we might get more bang for our buck if we could somehow get substantially better teachers, rather than more of them, as would be necessary to reduce class sizes. But the sad, often unstated truth about teacher quality is that there is very little evidence, at least as yet, that public policy can be used to improve it, whether cost-effectively or otherwise.
Positing teacher quality as a concrete policy intervention represents circular reasoning. It’s saying that, if we had more teachers who increase test scores, this would increase test scores. Well, yes. But that’s more of an effect than a means. The relevant policy question is: How do we do so?
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