In applied statistical work we rarely have the opportunity to measure entire populations, instead we make inferences about the populations from samples drawn, ideally randomly from the population. We usually want to know averages from whole populations, but use the average from a sample as our best guess at the population average. The unknown population average is called a parameter. The known sample average is called a statistic.
As a matter of convention, most statistical texts use greek symbols for population parameters and the English or Arabic letters for sample statistics. For example, a sample mean is often denoted as x-bar . Population means are denoted with the lower-case greek letter mu μ .
