Math & Statistics

What Is the Interquartile Range (IQR)?

The interquartile range (IQR) is the difference between the third quartile (Q3) and the first quartile (Q1). It measures the spread of the middle 50% of a data set and, unlike the full range, is not affected by extreme values.

Outlier Rule

A common rule treats values below Q1 − 1.5 × IQR or above Q3 + 1.5 × IQR as potential outliers.

Example

With Q1 = 5 and Q3 = 13, IQR = 8; values below −7 or above 25 would be flagged.

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🔗 Related Terms

Quartile Range (Statistics) Outlier Standard Deviation

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Last updated: September 29, 2026. This definition is for general educational purposes only. It is not financial, investment, tax, legal, or medical advice, and it does not recommend any product or course of action. For decisions about your own situation, consult a qualified professional.