# Applying a post hoc Wilcoxon signed rank test to a Friedman when there are different sample sizes

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

• Friedman test is not the extension of Wilcoxon test on several related samples (it is closer to be the extension of sign test). So, it is very questionnable whether post-hoc pairwise comparisons corresponding to Friedman may be Wilcoxon tests. Dec 28, 2013 at 15:41
• +1 @ttnphns. However in this particular setting with so many drop outs, the choice between post-hoc sign-tests or post-hoc Wilcoxon sign rank tests seems to be a minor problem. Dec 29, 2013 at 11:30

• Why do you multiply the significance level by a factor of $(m-1) \cdot m$? Dec 29, 2013 at 15:11