School factors are not important: evidence from classroom separation studies http://emilkirkegaard.dk/en/?p=7395
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I've heard lots of "mainstream" sources claim large (~1 sigma!) differences in effectiveness between good and bad teachers on performance. Is this not true, or is it just that in practice there's not so much variance in teacher quality?
I'm sure there are systematic large differences between teachers, but these evidently don't translate into notable effects on students.