Standard Deviation is a measure of how spread out numbers are.

It is the square root of the Variance. It is often more convenient than the variance because it has the same unit as the data points.

**Population Standard Deviation**, $\sigma=\sqrt{\frac{1}{n}}\sum_i^n(x_{i}-\mu)^2$

**Sample Standard Deviation**, $\sigma=\sqrt{\frac{1}{1-n}}\sum_i^n(x_{i}-\overline{x})^2$