Bloom’s 2 sigma problem refers to an educational phenomenon and associated problem observed by educational psychologist Benjamin Bloom [Benjamin Bloom] and initially reported in 1984 in the journal Educational Researcher. Bloom found that the average student tutored one-to-one using mastery learning [Mastery learning] techniques performed two standard deviations better than students educated in a classroom environment with one teacher to 30 students, with or without mastery learning. As quoted by Bloom: “the average tutored student was above 98% of the students in the control class”. Additionally, the variation of the students' achievement changed: “about 90% of the tutored students … attained the level of summative achievement reached by only the highest 20%” of the control class.
(“Bloom’s 2 Sigma Problem” 2022)