Interquartile Range

Divide a set of data into two groups (high and low) of equal size at the Median if there is an Even number of data points, or two groups consisting of points on either side of the Median itself plus the Median if there is an Odd number of data points. Find the Medians of the low and high groups, denoting these first and third quartiles by $Q_1$ and $Q_3$. The interquartile range is then defined by

$${\rm IQR}\equiv Q_3-Q_1.$$

See also H-Spread, Hinge, Median (Statistics)