RM-ANOVA vs. Nonparametric Tests
I have a design with a pretest and two posttests testing multiple questionnaires and experiments.
In this case, I was interested in the outcome of perceived stress scores (PSS). Please note that group sizes are unequal (35, 33), as well as there is some missing data along the three timepoints.
So I ran an RM ANOVA and got a significant measurement x group interaction (yeey!), but looking at the descriptives, Shapiro-Wilk tests for all three timepoints were p < .05. I was not sure whether I should ditch the ANOVA and run a Friedman's test, but I tried anyway. When I did that, I got an error message stating "Specific ANOVA design is not balanced".
How to continue? Any alternatives? :)
Many thanks for your answers!