Outlier Calculator
Two common rules for flagging values that sit unusually far from the rest.
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The formula
lower fence = Q1 − k × IQR upper fence = Q3 + k × IQR z = (x − mean) ÷ sample standard deviation
Tukey’s rule uses quartiles, which extreme values barely affect. The z-score rule uses the mean and standard deviation, which an outlier inflates.
What to watch out for
The z-score rule suits roughly bell-shaped data. Tukey’s fences make no such assumption, though on skewed data they will flag many values on the long side as a matter of course.
Frequently Asked Questions
What is the 1.5 × IQR rule?
A value is an outlier if it lies more than 1.5 IQRs beyond a quartile. With Q1 of 10 and Q3 of 20, the fences are -5 and 35.
Should I delete outliers?
Only with a reason, such as a measurement error. If you do remove any, report it.