The interquartile range (IQR) is a statistical measure that represents the spread of the middle 50% of a data set. It is calculated by subtracting the first quartile (Q1) from the third quartile (Q3). The first quartile is the value below which 25% of the data falls, while the third quartile is the value below which 75% of the data falls. The IQR is useful for identifying outliers and understanding the variability of the data. A larger IQR indicates a wider spread of values, while a smaller IQR suggests that the data points are closer together. This measure is particularly helpful in box plots, which visually display the distribution of data.