Because gathering data for entire populations is often impractical, most of the data we analyze are samples. In order to make the sample standard deviation more closely estimate the population standard deviation, statisticians modify the way they calculate the standard deviation for a sample.
Refer to the Math Notes box in Lesson C.1.3 in Appendix C to remind you of how to calculate standard deviation (for a population). The calculation of standard deviation of a sample is slightly different. When computing the mean of the distances squared, instead of dividing by the number of values in the data set, divide by one less than the number of values in the set.
Find the mean.
Find the distances to the mean from each data point.
Square the distances.
Find the mean of the distances squared.
Since this is a sample, divide by one less than the number of data items.
Square root your answer.