(R) Friedman Test (SPSS)

In previous articles, we discussed the concept of non-parametric tests. In the event that you are just tuning in, or you are un-familiar with the term “non-parametric test”, I will provide you with a brief conceptual overview.

What is a non-parametric test?

A non-parametric test is a method of analysis which is utilized to analyze sets of data which do not comply with a specific distribution type. As a result of such, this particular type of test is by design, more robust.

Many tests require as a prerequisite, that the underlying data be structured in a certain manner. However, typically these requirements do not significantly cause test results to be adversely impacted, as many tests which are parametric in nature, have a good deal of robustness included within their models.

Therefore, though I believe that it is important to be familiar with tests of this particular type, I would typically recommend performing their parametric alternatives. The reason for this recommendation relates to the general acceptance and greater familiarity that the parametric tests provide.

Friedman Test

The Friedman Test provides a non-parametric alternative to the one way repeated measures analysis of variance (ANOVA). Like many other non-parametric tests, it utilizes a ranking system to increase the robustness of measurement. In this manner, the test is similar to tests such as the Kruskal-Wallis test. This method of analysis was originally derived by Milton Friedman, an American economist.

Example

Below is our sample data set:

To begin, select “Analyze”, “Nonparametric Tests”, followed by “Legacy Dialogs”, and then “K Related Samples”.

This populates the following screen:

Through the utilization of the center arrow button, designate “Measure1”, “Measure2”, and “Measure3” as “Test Variables”. Make sure that the box “Friedman” is selected beneath the text which reads: “Test Type”.

After clicking “OK”, the following output should be produced:

Since the “Asymp. Sig.” (p-value) equals .667, we will not reject the null hypothesis (.667 > .05). Therefore, we can conclude, that the three conditions did not significantly differ. 

As a reference, if the original data was analyzed within SPSS through the utilization of a repeated measures analysis of variance (ANOVA) model, the probability outcome would be incredibly similar. 

To perform this test within R, we will utilize the following code:

# Create the data vectors to populate each group #

Measure1 <- c(7.00, 5.00, 16.00, 3.00, 19.00, 10.00, 16.00, 9.00, 10.00, 18.00, 6.00, 12.00)

Measure2 <- c(14.00, 12.00, 8.00, 17.00, 18.00, 7.00, 16.00, 10.00, 16.00, 9.00, 10.00, 8.00)

Measure3 <- c(9.00, 13.00, 3.00, 6.00, 2.00, 16.00, 15.00, 7.00, 13.00, 17.00, 9.00, 13.00)

# Create the data matrix necessary to perform the analysis #

results <-matrix(c(Measure1, Measure2, Measure3),
ncol = 3,
dimnames = list(1 : 12,
c("Measure1", "Measure2", "Measure3")))

# Perform the analysis through the utilization of The Friedman Test #

friedman.test(results)

This produces the output:

Friedman rank sum test

data: results
Friedman chi-squared = 0.80851, df = 2, p-value = 0.6675

That’s it for this article, stay tuned for more. Never stop learning, Data Heads!