e(X)= X 1 * p(X 1)+X2 * p(X2)+……+Xn * p(Xn)= X 1 * f 1(X 1)+X2 * F2(X2)+……+Xn * fn(Xn)。

x； 1，X； 2，X； 3、……、X .

N is a discrete random variable, and p(X 1), p(X2), p(X3), ... p (xn) are the probability functions of these data. P(X 1), p(X2), p(X3), ... p (Xn) in several random data is understood as the frequency f(Xn), X2, X3, ..., xn of data X 1.

Variance of common distribution

1, two points distribution.

2. binomial distribution? X ~ B (n, p) introduces a random variable Xi (the number of times a appears in the first experiment follows a two-point distribution).

3. Poisson distribution (deduction omitted).

4. evenly distributed? Another calculation process is.

5. Exponential distribution (omitted).

6. Normal distribution (omitted).

7.T distribution: where X~T(n) and E(X)=0.

8.F distribution: where X~F(m, n).