3 Sigma(σ) Rule, 68–95–99.7 Rule, Empirial Rule

https://en.wikipedia.org/wiki/68%E2%80%9395%E2%80%9399.7_rule

In statistics, the 68-95-99.7 rule, also known as the empirical rule, is a shorthand used to remember the percentage of values that lie within an interval estimate in a normal distribution: 68%, 95%, and 99.7% of the values lie within one, two, and three standard deviations(σ) of the mean, respectively.

https://ko.wikipedia.org/wiki/68-95-99.7_%EA%B7%9C%EC%B9%99