Definition

Standard deviation

A measure of how spread out values are around their mean, in the same units as the data.

The standard deviation (SD) summarizes variability: in roughly normal data, about 68% of values fall within one SD of the mean and 95% within two. Two datasets can share a mean and differ wildly in SD — which is why reporting means without SDs hides half the story.

SD describes the data; standard error describes the precision of the mean estimated from it. Confusing the two — or error bars that do not say which they show — is among the most common figure problems reviewers flag.