 Thus formula_24, and therefore formula_25 is an unbiased estimator of the population variance, "σ"2.The ratio between the biased (uncorrected) and unbiased estimates of the variance is known as Bessel's correction.The reason that an uncorrected sample variance, "S"2, is biased stems from the fact that the sample mean is an ordinary least squares (OLS) estimator for "μ"