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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Why is there a conflict? Is there evidence that suggests it does produce a two sigma gain.
Or are you basing it off priors from athletics and music?
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It's a classic (though inconsistently replicated) result from Bloom that's cited as a motivator for many actors in the edu space.
en.wikipedia.org/wiki/Bloom%27s
"average student tutored one-to-one using mastery learning techniques performed two standard deviations better than students who learn via conventional instructional methods"
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