PATRICK COLLISON ASKS, "Is Bloom's Two Sigma phenomenon real? If so, what do we do about it?" 
One of Patrick's Questions pointed to educational psychologist Benjamin Bloom who found one-on-one tutoring using mastery learning led to a two sigma improvement in student performance. The results were replicated.  Bloom asks in his paper that identified the "2 Sigma Problem": how do we achieve these results in conditions more practical (i.e., more scalable) than one-to-one tutoring?