The 2-sigma challenge refers to Bloom's research finding that one on one tutoring using mastery learning lead to a 2 sigma improvement in student performance. T

The 2-sigma challenge refers to Bloom's research finding that one on one tutoring using mastery learning lead to a 2 sigma improvement in student performance. The article is in part a discussion of whether this conclusion is justified. I love this quote from the article, perhaps because it suggests that things could get a lot better at our schools without miraculous or infeasibly expensive interventions by focusing on reliable smaller effects. The 2-sigma challenge (or 1-sigma claim) is misleading out of context and potentially damaging to educational research both within and outside of the mastery learning tradition, as it may lead researchers to belittle true, replicable, and generalizable achievement effects in the more realistic range of 20-50% of an individual-level standard deviation. For example, an educational intervention that produced a reliable gain of .33 each year could, if applied to lower class schools, wipe out the typical achievement gap between lower and middle-class children in 3 years—no small accomplishment. Yet the claims for huge effects made by Bloom and others could lead researchers who find effect sizes of "only" .33 to question the value of their methods.