Where x represents the average of samples, n represents the number of samples, xi represents individuals, and S 2 represents variance.
The concept and calculation formula of variance, for example, their scores in five exams are as follows: x: 50, 100, 100, 60, 50, and the average e (x) = 72; Y: 73, 70, 75, 72, 70 Average E(Y)=72. The average score is the same, but x is unstable and deviates greatly from the average. Variance describes the deviation between random variables and mathematical expectations.
Extended data
Square difference: a? -B? =(a+b)(a-b). Text expression: The product of the sum of two numbers and the difference of two numbers is equal to the square of the difference of two numbers. This is the square difference formula.
Standard deviation: standard deviation = sqrt ((x1-x) 2+(x2-x) 2+... (xn-x) 2)/n). Is the square root of the arithmetic mean of the mean square deviation, expressed by σ.