The interquartile range (IQR) is a resistant measure of spread for a quantitative data set. A ‘resistant measure’ is not influenced by outliers. A ‘measure of spread’ indicates how consistent or variable the data set is. Other measures of spread that are not resistant to outliers are the standard deviation and the range (maximum – minimum). The IQR measures the number of units in which the middle 50% of the data lie and is used in one technique to determine outliers in a data set.

The IQR is the difference between the third and first quartiles, Q3 and Q1, respectively, that is, IQR = Q3 − Q1. The IQR is always a positive value. The Q3 value is the 75th percentile, while Q1 is the 25th ...