How is the Standard Deviation calculated?

It is the
standard deviation of the monthly returns of the fund in the specified time
period, for example 3 years. Since the underlying monthly returns are expressed
as percentage points, so are the units of the standard deviation.

For example, if a fund has a mean (average) return of 10% and a standard
deviation of 5%, there is a 68% probability that the fund's return will be
between 5% and 15% (10% plus/minus 5%). According to statistical theory, there
is a 95% probability that the fund's return will be within two standard
deviations, in this case between 0% and 20% (10% plus/minus 2 times 5% or 10%).