I conducted a Friedman test to related-sample group in three conditions (pre-treatment, post-treatment and 6-month post treatment). Out of 216 participants that completed the first two stages, 50 of these completed the 6-month post stage. Am I right in thinking that the Friedman test is doing the calculation on the 50 participants that completed all stages?
My problem is that I then want to conduct a Wilcoxon signed ranks post hoc test to examine where the differences are. N = 216 participants as opposed to the 50 used in the Friedman test.
Could anyone tell me if it is ok to use this result? Or do I have to separate the data further so the Wilcoxon test uses the 50 participants? Does this impact its validity?
Any help would be greatly appreciated,
Sam
Best Answer
For post-hoc test you only should use those data that you previously used in Friedman, so all three pairwise compared groups should be of the sample size 50. The critical value of the test should also be adjusted by factor (m-1) where m is the number of groups. Holding the level of the type I error is at the expense of the power of the test.