Quote by Elucidus
Interestingly enough the use of the divisor (n  1) is a rather recent development. I've read texts as recent as the 40's where the standard deviation has a devisor of n. As such the expected value of s^{2} is [itex]n^2\cdot \sigma^2 / (n1)^2[/itex] if I'm not mistaken. Since is was an underestimator of the true variance, statisticians and probabilists switched to the current divisor of (n  1).
Whether the divisor is n or (n  1), these expressions are all measures of dispersion; whether they're good measures is another matter.
Elucidus

For large n it doesn't really matter.