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Wilcoxon Signed-Rank Test in SPSS 

Wilcoxon Signed-Rank Test in SPSS 

The Wilcoxon Signed-Rank test is a non-parametric test that is equivalent of the paired t test. 

The Wilcoxon Signed-Rank test has the following hypotheses.

Null hypothesis: Median of the population of differences between the paired data is zero. Alternative hypothesis: Median of the population of differences between the paired data is not equal to zero.

Assumptions

  1. Variables should be continuous.

  2. Observations should be independent.

  3. Population distribution of the difference scores must be symmetric.

Systolic blood pressure for the group of patients was measured both before and after the treatment. Here, an attempt is being made to find if there is any difference in systolic blood pressure before and after the treatment.

In SPSS, Wilcoxon Signed-Rank Test is found in Analyze > Nonparametric Tests > Legacy Dialogs > 2 Related Samples

Then, we will get ‘Two Related-Samples Tests’ dialog box.

Add before treatment in the ‘Variable1’ box and add after treatment in the ‘Variable2’ box. Then, tick the ‘Wilcoxon’ option in the ‘Test Type’ group.

In the options dialog box, click on ‘Descriptive’ box under the ‘Statistics’ group. Then, click on Continue and Ok.

Output of Wilcoxon Signed-Rank Test

Consequently, ‘Descriptive Statistics’ table would be obtained and this table gives values of mean, standard deviation, minimum and maximum for the two variables.

Then, ‘Ranks’ table would be obtained. This table gives data for comparing patients’ systolic blood pressure values before and after the treatment. It can be inferred from the table that 28 patients had high systolic blood pressure values before treatment when compared to those after treatment. Nine patients had high systolic blood pressure values after treatment when compared to those before treatment. Five patients had the same systolic blood pressure values both before and after treatment.

Then, we will get ‘Test Statistics’ table.

Here, p value (Asymp. Sig. (2-tailed)) is equal to 0.000. Thus, it can be concluded that a significant difference exists in systolic blood pressure values of patients before and after treatment.

Wilcoxon signed-rank test determined that there was a statistically significant median decrease in systolic blood pressure when the patients were treated (Z=-3.695, p=0.000).

Data: Wilcoxon_Signed-Rank_Test_data.sav

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