Interquartile range ($\mathrm{IQR}=Q_3-Q_1$)

The interquartile range measures the width between the lower and upper quartiles, in the same units as the observations. It describes central spread rather than the distance between the observed extremes. In our toy box plot, $\mathrm{IQR}=6.25-2.75=3.5$; moving the largest value from $20$ to $200$ would leave the quartiles and interquartile range unchanged under the chosen rule, because the order statistics used in their interpolation would not change.