MATHEMATICAL EXPECTATION The mathematical expectation of a variable, Yi, written E[Yi], is the population average of this variable. If Yi is a variable generated by a random process, such as throwing a die, E[Yi] is the average in infinitely many repetitions of this process. If Yi is a variable that comes from a sample survey, E[Yi] is the average obtained if everyone in the population from which the sample is drawn were to be enumerated.
