translation.en: The mastery-based population was a full standard deviation, or sigma, in achievement scores better than the standard lecture-based class, and the individual tutoring gives you 2 sigma improvement in performance.
translation.zh: 旨在精通的人群是完全标准差 或者sigma,他们在成就分数方面 比标准的以授课为基础的班要好得多 而导师一对一的课程在表现提高方面 就有2个sigma