Math & Statistics

What Is an Outlier in Statistics?

An outlier is a data point that is unusually far from the other values in a data set. Outliers can come from measurement errors, data entry mistakes, or genuine but rare observations. They can strongly affect the mean and range, while the median is much less sensitive to them.

Common Detection Rules

The 1.5 × IQR rule flags values more than 1.5 interquartile ranges below the first quartile or above the third quartile. Another approach flags values with a z-score beyond about ±3.

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🔗 Related Terms

Median Mean (Average) Z-Score Range (Statistics)

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Last updated: September 27, 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.