Standard deviation
The answer is the population standard deviation. The formula is only true if the eight numbers we started with are the whole group. If they are only a part of the group picked at random, then we can obtain an unbiased estimate of what the population standard deviation is by dividing by  instead of  in the bottom (denominator) of the formula above. Then the answer is the (bias-corrected) sample standard deviation. This is called the Bessel's Correction. We often use this correction because the sample variance, i.e., the square of the sample standard deviation, is an unbiased estimator of the population variance, in other words, the expected value or long-run average of the sample variance equals the population (true) variance. However, it is not the case that the sample standard deviation is an unbiased estimator of the population standard deviation. Although Bessel's correction is an unbiased estimate of the variance, this estimate does have a higher mean square error than the biased estimate, or in other words, the biased estimate (that is, dividing by n rather than n-1) is on average closer to the true value.