Monday, February 25, 2013

What's the Mechanism by which Having a Good Teacher Turns into a 1% Income Gain?

Kevin Drum asks whether the estimated 1% subsequent income gain for a student from having a bottom 5% teacher measured by VAM replaced with an average one sounds like a surprisingly big or small effect.

I think it is a pretty small effect, but seems about right intuitively. What's impressive is that they could tease it out statistically at all.

I've wondered about by what mechanism this difference actually occurrs, though. My guess is that for the vast majority of students there is no effect at all but for some children a good or bad teacher would cause them to be sorted into a higher or lower path, particularly in math, and in the case of the higher path, having the necessary skills to be successful on that path. That could result in a much more than a 1% difference in income, for the 1 kid in 10, say, who would end up going to college because he was bumped out of the remedial math group in third grade, which would then average out as a 1% difference over the entire sample.

I'm neither sufficiently curious nor conversant in statistics to try to figure out if they observed that distribution.

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