The interquartile range (IQR) is a measure of the spread within a numerical data set. It is equal to the upper quartile (Q₃) minus the lower quartiles (Q₁); that is, IQR = Q₃ – Q₁

The IQR is the width of an interval that contains the middle 50% (approximately) of the data values. To be exactly 50%, the sample size must be a multiple of four.