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Grubbs test to detect outliers in Excel tutorial


This tutorial will help you run a Grubbs test to detect outliers and interpret the results in Excel using XLSTAT.
Not sure this is the statistical test you are looking for? Check out this guide.

Dataset for testing outliers with Grubbs test

An Excel sheet with both the data and the results can be downloaded by clicking here.

The data have been obtained from a normal distribution with mean 0 and variance 3. One outlier has been added. We wish to test if there is one outlier in the sample.

Goal of this tutorial

We would like to detect an outlier from a sample using Grubbs test. 

Setting up a Grubbs test to detect an outlier

To start the Grubbs test go to the menu Testing outliers / Grubbs test.

grubbs test menu

In the General tab, select the data and the Grubbs test option (the Double Grubbs test can be used to detect two outliers).

Grubbs test general tab XLSTAT

As an alternative hypothesis choose the two-sided option. The default significance level is left as is: 5%.

grubbs test dialog box options

When ready click on OK.

Results of a test for detecting an outlier

The result is that the p-value for this test is  slaller than 0.0001. That means that the null hypothesis should be rejected.

grubbs test result

In the following table, the detected outlier is given.

grubbs test outlier

You can also find the Z score to detect outliers in the output of this test.

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