3.97 FAQ-490 How do I identify and eliminate outliers?

Last Update: 8/7/2015

For a dataset from repeated measurements, generating a Q-Q plot would help to visualize outliers.

A Q-Q plot is a graphic method for testing whether a dataset follows a given distribution, but it may also be used to determine outliers.

Outlier Q Q Plot 001.png

The expected values are not following the reference line, indicating the data was not normally distributed, the data points away from the reference lines are suspected outliers.

One should remove or mask the suspected outliers from the Q-Q plot, and when the expected values follow the reference line, all outliers are eliminated.

Outlier Q Q Plot 002.png

Notes: To test for outliers or determine outlier in a regression, refer to the Detecting Outliers.

Keywords:outlier, Q-Q plot, probability plot, mask, detect outlier