This formula is valid only if the eight values with which we began form the complete population. If the values instead were a random sample drawn from some large parent population (for example, they were 8 students randomly and independently chosen from a class of 2 million), then one divides by instead of in the denominator of the last formula, and the result is formula_6 In that case, the result of the original formula would be called the "sample" standard deviation and denoted by "s" instead of formula_7 Dividing by "n" − 1 rather than by "n" gives an unbiased estimate of the variance of the larger parent population. This is known as "Bessel's correction". Roughly, the reason for it is that the formula for the sample variance relies on computing differences of observations from the sample mean, and the sample mean itself was constructed to be as close as possible to the observations, so just dividing by "n" would underestimate the variability.