A plot of a normal distribution (or bell curve). Each colored band has a width of one standard deviation.

A data set with a mean of 50 (shown in blue) and a standard deviation (σ) of 20.

Example of two sample populations with the same mean and different standard deviations. Red population has mean 100 and SD 10; blue population has mean 100 and SD 50.

Standard deviation is a number used to tell how measurements for a group are spread out from the average (mean or expected value). A low standard deviation means that most of the numbers are close to the average, while a high standard deviation means that the numbers are more spread out.