The standard deviation is a commonly used measure of spread of a data set. It is a measure of how much the data elements differ from the mean. To denote the standard deviation, the Greek letter $\sigma$ is commonly used, which is read as sigma. The standard deviation is the quadratic mean of the difference between the data values $x_i$ and the mean $\bar{x}.$ $$\sigma =\sqrt{\frac{(x_1-\bar{x}{)}^2+(x_2-\bar{x}{)}^2+\dots +(x_n-\bar{x}{)}^2}{n}}$$ The applet below calculates the standard deviation for the data set on the number line. Move the points around to change the data.