In statistics and probability theory, the Greek letter "mu" (μ) is used to denote the population mean or expected value. The population mean is the average score of the population on a given variable. The formula for the population mean is μ = ( Σ Xi ) / N, where Σ Xi represents the sum of all scores present in the population, and N represents the total number of individuals or cases in the population. The observed value of mu will be simply μ. In journal articles, the mean is usually represented by M, and the median by Mdn.