The expected value, or expectation, of a random variable, is merely its average value, where we speak of "average" value as one weighted according to the probability distribution. The expected value of a distribution can be thought of as a measure of center, as we think of averages as being middle values. By weighing the values of the random variable according to the probability distribution, we hope to obtain a number that summarizes a typical or expected value of an observation of the random variable.