Interquartile range (IQR)

The interquartile range measures how spread out the middle 50 percent of the data is, so it ignores the extreme quarters. You compute it as $\text{IQR} = Q_3 - Q_1$, the third quartile minus the first quartile. For example, if $Q_1 = 3$ and $Q_3 = 9$, then the IQR is $9 - 3 = 6$. Because it drops the top and bottom quarters, the IQR is resistant to outliers and drives the 1.5 times IQR rule for flagging them.