It gives an indication of the spread in the values: are they all very close to the mean value or scattered across a wide range of values? That is important in determining how accurate your point estimate of the mean is.
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Square the standard deviation and you will have the variance.
You need more than one number to calculate a standard deviation, so 9 does not have a standard deviation.
Standard deviation = square root of variance.
Standard deviation is how much a group deviates from the whole. In order to calculate standard deviation, you must know the mean.
In the same way that you calculate mean and median that are greater than the standard deviation!