Standard deviation measures how spread out values in a dataset are around the mean. There are two versions of the formula: population standard deviation (σ) for complete datasets, and sample standard deviation (s) for subsets drawn from a larger population. The key difference is the denominator: N for population (divide by total count) vs n−1 for samples (Bessel's correction to reduce bias). This page explains both formulas with step-by-step calculations and explains when to use each.