Wilcoxon Signed Ranks Test

 

1 Continuous Dependent Variable with non-normal distribution

1 (2 level) Categorical Independent Variable

 

 

 

 

 

 


The Wilcoxon Signed Ranks test is the non-parametric version of the 2 independent samples t-test, which means the test is appropriate when you want to conduct a 2 independent samples t-test, but the dependent variable is not normally distributed.

 

Rodden and Wood use the Wilcoxon Signed Ranks test when studying how people manage their digital and non-digital photographs [10]. The experiment took place over a 6 month period. At the beginning of the study, subjects answered questions about how they organized and used their non-digital photo collection. Subjects were then given digital cameras, and used them for 6 months. At the conclusion of the study, subjects described how they organized and used their digital photographs. The research team compared their habits as follows:

           

Statement

Mean

Median

“I am content with the way my photo collection is organized”

N

D

2.2

4.5

2

5

“I browse my photo collection often”

N

D

1.8

2.9

2

3

“When I am looking for something in particular, it is easy for me to find it”

N

D

1.9

4.6

2

2

 

Table 1: Comparing participants’ opinions about their non-digital (N, from the first interview) and digital (D, from the second interview) collections.

From Rodden and Wood [10].

 

Rodden and Wood demonstrate a valid application of the Wilcoxon Signed Ranks test. First, their experiment included 13 subjects, and the test is appropriate for small sample sizes. Second, the experiment measured a dichotomous independent variable: organization of digital verses non-digital photographs. Finally, the dependent variable, answers to question in Table 1, were measured on 7-point likert scale, which is an interval scale, and appropriate for the Wilcoxon Signed Ranks test. The research team conducted 3 Wilcoxon Signed Ranks tests to evaluate the data in Table 1, one for each question. The results were significant for all three questions, so the study reveals individuals do report different habits for managing digital and non-digital photographs.

 

Values to report:

·        Mean value of each test group

·        Z value

·        p value