Gabriel Poesia

The Two Sigma Problem (@ Educational Researcher 1984)

Benjamin Bloom


This is the classical paper by educational psychologist Benjamin Bloom, where he formulated the so-called "Two-Sigma Problem". Essentially, the problem is that 1:1 tutoring is an extremely effective learning setting: students that undergo 1:1 tutoring with good tutors have an *average* achievement of two standard deviations above the conventional setting of group instruction in a classroom. The problem is that 1:1 tutoring is very expensive, and thus an exclusionary mode of instruction that is only accessible to richer families. How, then, can we make group instruction as effective? This is the challenge that he poses.

I've heard that this result was not consistenly replicated by later research, with effect sizes tending to be smaller than 2 sigma (which is a lot). Even if smaller, the result is still significant, and on another level just very intuitively true: good 1:1 tutors can pay much more attention to the individual student's needs, adapt the pace, go back whenever necessary, and so on. The paper surveys their research on a method of "Mastery Learning", that is an attempt to bridge this gap in a classroom setting.

There's hope by many that technology can eventually be useful towards this goal. However, this story is old. So far, edtech has promised more than it has delivered in terms of promoting equity at scale.