## Quick Reference

The average value of some variable that is measured for all members of a (possibly infinite) population. If the value of the variable for a randomly chosen member of the population is denoted by *X*, then the population mean is the expectation of *X* and is usually denoted by *μ* (the notation *μ* was introduced in 1936 by Sir Ronald Fisher in the sixth edition of his Statistical Methods for Research Workers). Similarly, the population variance, usually denoted by *σ*^{2}, is the mean of the squared differences between the values of the members of the population and the population mean: this is the expectation of (*X*−*μ*)^{2}.

*Subjects:*
Probability and Statistics.

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