Bloom’s Two Sigma Challenge

Benjamin Bloom (well-known for Bloom’s Taxonomy), coined the term Mastery Learning when he observed that most children can indeed fully grasp knowledge and gain strong command of skills, if allowed to engage in guided deep practice as they learn, and if permitted to spend as much time as they need on each skill before moving on to the next. 

Comparing Tutoring to Conventional Classrooms

Importantly, Bloom’s research then revealed that children who experience mastery learning under the guidance of a one-to-one tutor, perform two standard deviations better than their peers in conventional classrooms. Given this stark disparity in likely learning outcomes of tutored children compared to those not tutored, Bloom urgently challenged educators and education systems to identify methods of group instruction that could be as effective as one-to-one mastery learning tutoring. This became known as Bloom’s Two Sigma Challenge. 

Many experiments have been inspired by Bloom’s findings. But most have been constrained by the reality that it is very difficult to scale traditional one-to-one tutoring. The costs of delivery, or the certainty of teacher variability (which compromises consistency in tutoring quality), or both, make any teacher-as-tutor model alone very challenging to replicate, scale, and sustain. 


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