The sample standard deviation of observations 
is the positive square
root of the sample variance,
where
is the sample mean. The divisor 
makes 
an unbiased estimator
of the population variance for independent
and identically distributed observations. The standard deviation 
itself is generally not unbiased for the population
standard deviation.
See also
Population Standard Deviation,
Sample Variance,
Standard
Deviation
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References
Casella, G. and Berger, R. L. Statistical
Inference, 2nd ed. Pacific Grove, CA: Duxbury, 2002.
Cite this as:
Weisstein, Eric W. "Sample Standard Deviation."
From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/SampleStandardDeviation.html
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