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• When do I use the Wilcoxon signed-rank test?
• Can I use the Wilcoxon signed-rank test to compare two visualization methods with each other?
• Where do I find the Wilcoxon signed-rank test in SPSS?
• References

The Wilcoxon signed-rank test is a non-parametric test for comparing a (semi-)continuous variable between two paired groups.

If you want to test whether the values of a twice-measured, paired, (semi-)continuous variable differ, you can use the Wilcoxon signed-rank test. For example, if you want to test whether the results of two CT scans of a patient differ from each other.

The Wilcoxon signed-rank test is often used as an alternative to the paired t-test, because the Wilcoxon signed-rank test does not assume normally distributed data. The test may always be used, for all ordinal data.

Example of the use of the Wilcoxon signed-rank test:

Table 1. Results of repeated CT scans.
Variable*	scan 1	scan 2	p-value**
CT measurement	1004 [565;1222]	1114 [668;1485]	0.03
*Variables are denoted as median [interquartile range]. **Differences between the two scans were tested with the Wilcoxon signed-rank test.

In the study I used 2 different methods in 73 patients to visualize tumors during surgery. I want to investigate whether method 1 can visualize abnormal/suspicious spots better than method 2. I therefore want to calculate whether method 1 visualizes significantly more spots than method 2. My proposal is to do this using the Wilcoxon signed-rank test. Is this the right test for this?

Do I understand correctly that you used both visualization methods in each patient and therefore have paired observations? The Wilcoxon signed-rank test then sounds like a good option for comparing the measured tumors with each other. You are then essentially comparing the median number of tumors per patient (instead of the mean, as with a t-test).

You will find the test in SPSS 28 under Analyze -> Non-parametric Tests -> Related Samples.

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