XLSTAT: Testing equivalence with TOST in Excel

This tutorial will help you run and interpret an equivalence test (TOST) to validate the difference between two means, in Excel using the XLSTAT software. Not sure this is the statistical test you are looking for? Check out this guide .

Background

The XLSTAT equivalence test allows to test if two products are equivalent. TOST means two-one-sided-test.

This type of test is used primarily to validate bioequivalence. When we want to show the equivalence of two drugs, classical hypothesis testing does not apply, we will use equivalence testing which will validate the equivalence between the two drugs.

XLSTAT offers two equivalent methods to test equivalence using the TOST test.

• Using the 100 * (1-2 * alpha) % confidence interval around the mean. By comparing this interval to the user-defined interval of equivalence, we can conclude the equivalence or non-equivalence. Thus, if the confidence interval is within the interval defined by the user, we conclude the equivalence between the two samples. If one of the bounds of the confidence interval is outside the interval defined by the user, then the two samples are not equivalent.
• Using two one-sided tests, one on the right and one on the left. We apply a right one-sided t-test on the lower bound of the interval defined by the user and a left one-sided t-test on the upper bound of the interval defined by the user. We obtain p-values for both tests. We take the greatest of these p-values as p-value of the equivalence test.

Dataset for Example

The data represent the time that a customer will spend in a restaurant depending on the type of wine that is offered (red or white). The restaurant owner wants to have his customer stay longer so that they spend more money in his establishment. There are 42 customers that have been studied.

We want to know if the fact that a customer chooses red or white wine is equivalent regarding the time spent in the restaurant. The difference should not exceed 10 minutes, we choose to have a TOST interval of [-10; 10].

Setting Up

• Open XLSTAT.
• Click on the Parametric tests menu and select the TOST (Equivalence test) feature:

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

In the General tab:

• In the Sample 1, select the White Wine variable.
• In the Sample 2, select the Red Wine variable.
• Check the Column labels.

In the Options tab:

• Enter the value 0 in the field for the difference to test, reflecting the null hypothesis of no difference.
• Set the lower bound of the equivalence interval to -10.
• Set the upper bound of the equivalence interval to 10.

• Click on the OK button to launch the computations.
• The results are displayed on a new Excel sheet.

Interpreting the Results

The first results displayed are the statistics for the various samples. Next, the equivalence test is displayed. There are two equivalent results to interpret the TOST.

The first table displays the 90% confidence interval of the mean and compares it to the user defined TOST interval.

In order to have equivalence between red and white wine, the 90% confidence interval has to be included in the TOST interval. We can see that -16,140<-10, so the equivalence is not accepted.

The second table is equivalent to the first one but using one-sided t-test with associated p-values.

We see that with a total p-value of 0.219, the equivalence is rejected.

We can conclude that offering white or red wine is not equivalent regarding the time spent in the restaurant by the customers.