Standard deviation
Also: SD, sample spread
A measure of how far observations typically spread around their mean.
Sample standard deviation is the square root of squared deviations divided by n−1; interpretation depends on sampling and distribution.
- Units
- same units as the measurement
- Formula
s = √(Σ(xᵢ−x̄)²/(n−1))
Example Repeated lengths may average 10 mm with s = 0.2 mm.
Common misconception Standard deviation is not accuracy and does not include every systematic error.