Standard deviation, or lower case sigma, measures the spread, or deviation, of data about a central tendency, such as the mean or the median in a data set or group, with a Gaussian (bell curve) distribution.

Standard deviation is a widely used measure of variability or diversity used in statistics and probability theory. It shows how much variation or "dispersion" exists from the average (mean, or expected value). A low standard deviation indicates that the data points tend to be very close to the mean, whereas high standard deviation indicates that the data points are spread out over a large range of values. Read more