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Student's t test on two independent samples tutorial

2017-03-22

This tutorial will help you compare two observed means (independent samples), using the two sample t-test and z-test, in Excel with XLSTAT.
Not sure this is the statistical test you are looking for? Check out this guide.

Dataset to run a Student's t test on two independent samples

An Excel sheet containing both the data and the results for use in this tutorial can be downloaded by clicking here.

The data are from [Fisher M. (1936), The Use of Multiple Measurements in Taxonomic Problems. Annals of Eugenics, 7, 179 -188] and correspond to 100 Iris flowers, described by four variables (sepal length, sepal width, petal length, petal width) and their species. The original dataset contains 150 flowers and 3 species, but we have isolated for this tutorials the observations belonging to the versicolor and virginica species. Our goal is to test if for the four variables, there is a clear dfference between the two species.

iris_versicolor.jpgiris_virginica.jpg

Iris versicolor and virginica.

Setting up a Student's t test on two independent samples

After opening XLSTAT, select the XLSTAT / Parametric tests / Two-sample t and z-test command, or click on the corresponding button of the Parametric tests toolbar (see below).

barttest.gif

Once you've clicked on the button, the dialog box appears.

The format of the dataset is such that you have one variable per column.

If we were running the test for one variable only (out of the four), we could also have one column for each specie. The third format is for paired samples (in that case, there must be one column per sample).

We choose to run only the t-test.

ttest1.gif

In the Options tab, we leave the default options. In the Charts tab, we activate the option for the dominance diagram.

ttest2.gif

The computations begin once you have clicked on OK. The results will then be displayed in a new sheet.

Interpreting the results of a Student's t test on two independent samples

The first results displayed are the statistics for the various samples. Next, the t tests and the dominance diagrams are displayed for each dimension one after the other.

The results for the first of the four variables are displayed below.

ttest3.gifttest4.gif

The difference appears to be very significant for the Sepal Length.

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