Philip P. answered 01/16/18
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The formula for the standard deviation is:
σ = [(1/N)SIGMA(x-μ)2]1/2
where
σ is the standard deviation
SIGMA is the sum of all of the (x-μ)2
x is a data point in the data set
μ is the mean
N is the number of data points in the data set
For the standard deviation to be zero, then (x-μ)2 must be zero for every x, which means that every data point equals the mean.