This document summarizes Peter Bloom's 1984 paper on the "2-Sigma Problem" regarding effective teaching of large groups. Bloom's study found that average students taught one-on-one (tutoring) performed 2 standard deviations above students in conventional large lectures (2-Sigma). Students in "Mastery Learning" classes, with formative testing, performed 1 standard deviation higher. The document then lists alterable educational variables by their effect size on student performance, with tutoring and feedback having the largest impact. Bloom aimed to solve the 2-Sigma problem by combining variables, though no single combination achieved tutoring levels of performance.