Bloom's 2 sigma problem

The Bloom's 2 Sigma problem is about a person named Benjamin Bloom who did a study. He wanted to know why some students were successful in school and some weren't. He found out that students who had a "tutor" or someone helping them with their studies were two standard deviations (a measure of how far away from the average someone is) above the rest of the students. In other words, having a tutor made students do two times better than the average student!