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Handbook of Descriptive Statistics/Measures of Statistical Variability/Interquartile range
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The Interquartile Range (IQR) is the range from the 25th to the 75th percentile of a distribution. It represents the "Middle Half" of the data and is a marker of variability or spread that is robust to outliers.
Wikipedia article: Interquartile Range
The IQR is a component of the Box plot, a useful graphical representation of distributions.