The square root of variance and probably the most commonly used measure of variation. The principal alternative is the interquartile range, which is more robust, but less mathematically tractable than the standard deviation (s.d., σ).

Used on pages 24, 30, 39, 41, 48, 50, 53, 82, 84, 107, 108

Also known as s.d.

### Links:

- Wikipedia: Standard deviation
- British Medical Journal: Mean and standard deviation
- Laerd Statistics: Standard Deviation
- MathsIsFun: Standard Deviation and Variance