## Population Standard Deviation $ \sigma = \sqrt{\frac{\sum (x_i - \mu)^2}{N}} $ - $N$ is the population size - $\mu$ is the population mean The Standard Deviation is the square root of the [[Variance]]. $\pm$ 1 / 2 / 3 SDs describe in which range 68 / 95 / 99.7% of all values of a normally distributed variable are found. Use [Sample Standard Deviation](Sample%20Standard%20Deviation.md) if your data is from a sample only.