A Wilcoxon test, also know as a Wilcoxon Signed-Rank Test, compares the meds and distributions of two related groups, such as comparing the difference between pre-intervention and post-intervention test results.
It is considered a non-parametric test and therefore it is suitable for non-parametric data. To check if your data is parametric, please check our dedicated guide: Parametric or Not Guide (PDF)
If your data is parametric you should consider using a Paired-Samples t-Test
1. You will need your data to be in two columns, one for each the of the two levels of your IV (in this case 'Before' and 'After').
2. Select the "T-Tests" option, drop down the menu and select "Paired Samples T-Test":
3. The Paired Samples T-Test options will open. Using the arrow button, move the two levels of your variable into the "Paired Variables" box. You will want to unselect "Student's" and select "Wilcoxon rank" instead.
Look through the options on this view for additional information you may want, here we have selected the addition of "Descriptives" to get the meds for the write up and also the "Descriptive plots" which can help with interpretation.
Jamovi produces the results as you add variables and tick options. If you have results that you don't need or find you have not selected all that you want, you can come back to the menu at any point to change your selection. For this Wilcoxon test, our results are given in two tables; an "Paired Samples T-Test" table and a "Descriptives" table, these are produced in APA style formatting.
These results may not give you all the results you would like for your write up. It is common for Wilcoxon results to be reported alongside the Interquartile Range (IQR) or quartiles. These can be produced in Jamovi under the "Exploration" tab, in the "Descriptives" menu.
This table shows a selection of descriptive statistics: the sample size (N), the mean of each level (Mean),the med for each level (Med) the standard deviation of each level (SD), and the standard error of each level (SE). Typically the N and Meds are reported from this table.
This table shows the specific test results; the Wilcoxon Statistics (W) and the p-value (p).
Here we recommend caution to ensure this is reported as a Wilcoxon Signed-Ranks test and not as a t-test.
Here is an example of how this could be reported, with the statistics in APA format:
Students’ test results were compared Before and After the intervention workshop. On average, students performed better After (Mdn = 73.00) the intervention than Before (Mdn = 71.00). A Wilcoxon Test indicated this improvement, was statistically significant, W = 1523, p = .005.
