ref:https://en.wikipedia.org/wiki/Variance#Sample_variance
ref2: https://en.wikipedia.org/wiki/Bessel%27s_correction#Proof_of_correctness_-_Alternate_3
ref:https://en.wikipedia.org/wiki/Variance#Sample_variance
ref2: https://en.wikipedia.org/wiki/Bessel%27s_correction#Proof_of_correctness_-_Alternate_3