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The **mean** is what we call the *average* in ordinary English, just the sum of all the observations divided by the number of observations. Once we have chosen this method of describing the communality of a DataSeT, we usually use the **StandardDeviation?** to describe how the observations differ. The *SD* (as we abbreviate it) is the **square root of the average of squared deviations from the mean**.

The **mean** is the only value about which the sum of squared deviations is a minimum. If you calculate the sum of squared deviations from any other measure of CentralTendancy, it will be larger than for the mean. This explains why the StandardDeviation? and the mean are usually cited together in statistical reports.

[RABeldin]