What is Interquartile range (IQR)?
Also known as: midspread
The difference between the third quartile (Q3) and the first quartile (Q1): IQR = Q3 − Q1. It shows how widely the middle 50 percent of ordered data is spread. The difference from the range (largest minus smallest): the IQR is hardly affected by very large or very small values at the extremes, so it is a more reliable measure of spread for unevenly distributed data. A rule widely used in practice: a value below Q1 − 1.5 × IQR or above Q3 + 1.5 × IQR is suspected of being an outlier. There are several methods for calculating quartiles, so software may give slightly different results; the report should state which method was used.