1. Standard deviation and variance
Suppose a set of sample data is x1, x2, x3, x4, ......xn, which 
represents the average of this set of data, so the average distance from the sample , which is the standard deviation S, is expressed as follows:
The square of the standard deviation is the variance, and the calculation formula for the variance is as follows:
Standard deviation and variance are both statistics that measure the degree of dispersion of a set of data. In actual operations, the smaller the standard deviation and variance, the smaller the degree of dispersion, that is, the more stable the data.