Variance is the square of variance of each variable value and the average of its mean, which is the most important method to measure the dispersion degree of numerical data.

Variance is used to measure the deviation between a random variable and its mathematical expectation (that is, the average value). The variance (sample variance) in statistics is the average of the sum of squares of the difference between each data and its average.

Variance of high school mathematics knowledge points;

1. Let c be a constant, then D(C)=0 (constant has no fluctuation).

2.D(CX)=C2D(X) (constant square extraction).

3. If X and Y are independent of each other, the first two terms are just D(X) and D(Y), and the third term is expanded when X and Y are independent of each other, so the third term is zero.