If two random variables X and Y are independent of each other, then E[(X-E(X))(Y-E(Y))]=0, so if the above mathematical expectation is not zero, then X and Y are not independent of each other, that is, there is a certain relationship between them.
If x and y are statistically independent, the covariance between them is 0, because two independent random variables satisfy E[XY]=E[X]E[Y].
However, the opposite is not the case. That is, if the covariance of x and y is 0, they are not necessarily statistically independent. Covariance Cov(X, y) is measured by multiplying the covariance of x by the covariance of y, and two random variables with covariance of 0 are called uncorrelated.
Extended data:
The estimation of expected value can be simply expressed as the past average income of the financial asset or investment group, or simulated by computer model, or determined according to inside information. When the expected rate of return of each asset is equal to the sum of the product of the rate of return and the probability of occurrence in each case.
The expected rate of return of the portfolio is equal to the weighted average of the expected rate of return of each asset in the portfolio, and the weight is the ratio of the asset value to the portfolio value.