3sigma原则 / 经验法则

在统计学中，经验法则是在正态分布中，距平均值小于一个标准差、二个标准差、三个标准差以内的百分比，更精确的数字是68.27%、95.45%及99.73%

3sigma Rule

"3sigma" or "three sigma" refers to a statistical concept that is used to measure the spread or variability of data.

In statistics, "sigma" (σ) is a symbol that represents the standard deviation(标准差), which is a measure of how much the data values in a set vary from the average or mean value.

"3sigma" refers to three times the standard deviation, or the range of values that fall within three standard deviations from the mean. This range is often used as a benchmark for determining whether a data point or observation is unusual or an outlier.

For example, in a normal distribution (also known as a bell curve(贝尔曲线)), approximately 99.7% of the data values fall within three standard deviations from the mean. Therefore, if a data point falls outside this range, it is considered unusual or unexpected.