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The Standard Deviation :: Population versus Sample Standard Deviation
Things to remember:
σ is the population standard deviation which we usually don't know.
s is the sample standard deviation which is an estimate of the unknown population standard deviation.
We divide the sum of squares by 1 less than the sample size to account for the error in estimation from the sample standard deviation (called the degrees of freedom).
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September 26, 2015  anonomous wrote:
Nobody explained why it is better to divide into n1 instead of n
March 15, 2009  Anthoyn wrote:
sample standard deviation (called the degrees of freedom). the df n1 should be earlier in the sentence otherwise it reads sort of like the sample standard deviation(ALSO called the degrees of freedom
February 17, 2009  Rodrigo Averna wrote:
Why did we didvide the sum of sqared values by " one less than the sample size" and not the sample size?