The sum of the products of all possible values xi of a discrete random variable and the corresponding probability P(=xi) is called mathematical expectation (set absolute convergence of series), which is recorded as E. If a random variable only takes a finite number of values, it is one of the most basic mathematical characteristics of a random variable. It reflects the average value of random variables. Also called expected value or average value. It is a generalization of simple arithmetic average, similar to weighted average.