If the average of x 1, x2, x3 ... xn is m.
Variance S2 =1/n [(x1-m) 2+(x2-m) 2+...+(xn-m) 2].
Variance is the average deviation from square, which is called standard deviation or mean square deviation. Variance describes the degree of fluctuation.
For continuous random variable X, if its definition domain is (a, b) and the probability density function is f(x), the formula for calculating the variance of continuous random variable X is d (x) = (x-μ) 2 f (x) dx.
Discrete type:
If a random variable only takes a finite number of values or can be listed in a certain order, its range of values is one or several finite or infinite intervals, such a random variable is called a discrete random variable. If a variable can take any real number in a certain interval, that is, the value of the variable can be continuous, then this random variable is called a continuous random variable.