D(x) refers to variance, and E(x) refers to expectation. Variance is a measure of dispersion when probability theory and statistical variance measure random variables or a set of data. Variance in probability theory is used to measure the deviation between random variables and their mathematical expectations (that is, the mean value).
In probability theory and mathematical statistics, mathematical expectation (or mean for short, or expectation) is the sum of the probabilities of every possible result multiplied by its result in the experiment, and it is one of the most basic mathematical characteristics. It reflects the average value of random variables.
Extended data:
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. Variance describes the dispersion degree between the value of random variable and its mathematical expectation. (The greater the standard deviation and variance, the greater the dispersion)
If the values of x are concentrated, the variance D(X) is small, and if the values of x are scattered, the variance D(X) is large.
Therefore, D(X) is a quantity to describe the dispersion degree of X and a scale to measure the dispersion degree of X..
Baidu encyclopedia-variance
Baidu Encyclopedia-Mathematical Expectation