Why is standard deviation also known as the root mean square deviation?

Standard deviation is also known as the root mean square deviation because it is the square root of the means of the square deviation from the arithmetic mean. In the calculation of standard deviation, first the arithmetic average is calculated, and the variations of various items from arithmetic averages are squared. The squared deviations are totalled and the sum is divided by the number of items. The square root of the resulting figure is the standard deviation of the series. The S.D. is denoted by the Greek letter. σ (Sigma) Symbalically