If tutors really do reliably produce two-sigma learning gains, then why aren't most top achievers the product of tutors? Yes, they're expensive, but perhaps "only" 2-3x private school; there are a lot of wealthy parents. Diminishing returns for high achievers?
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Is there any reason to believe the two sigma result holds up?
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It mostly hasn’t, at least not at that scale, but contemporary studies do still typically find very large effects.
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Do you have some pointers to more recent studies?
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