Bloom’s Two-Sigma Problem

by Justin Skycak on

The average tutored student performed better than 98% of students in the traditional class.

In 1984, educational psychologist Benjamin Bloom published a landmark study comparing the effectiveness of one-on-one tutoring and traditional classroom teaching.

The difference was monumental: the average tutored student performed better than 98% of the students in a traditional class.

This finding led to a challenge widely known as Bloom’s two-sigma problem: can we develop methods of group instruction that are as effective as one-on-one tutoring?

(The terminology “two-sigma” comes from statistics, where the effects of interventions are often measured in standard deviations or sigmas. An effect size of 98% is slightly more than two sigmas.)

To quote Bloom directly:

  • "...[T]the most striking of the findings is that under the best learning conditions we can devise (tutoring), the average student is 2 sigma above the average control student taught under conventional group methods of instruction.

    The tutoring process demonstrates that most of the students do have the potential to reach this high level of learning. I believe that an important task of research and instruction is to seek ways of accomplishing this under more practical and realistic conditions than the one-to-one tutoring, which is too costly for most societies to bear on a large scale.

    This is the '2 sigma' problem. Can researchers and teachers devise teaching-learning conditions that will enable the majority of students under group instruction to attain levels of achievement that at present can be reached only under good tutoring conditions?
    If the research on the 2 sigma problem yields practical methods ... it would be an educational contribution of the greatest magnitude. It would change popular notions about human potential and would have significant effects on what the schools can and should do with the educational years each society requires of its young people."

Bloom speculated that an equivalent two-sigma effect might be achieved by combining various evidence-based learning strategies, especially those involving different objects of change (the learner, the instructional material, the home environment or peer group, and the teacher and teaching process) and those that occur at different times in the teaching-learning process.