I am trying to evaluate the usability of three prototypes of an application and want to see if there is a significant difference in terms of usability. For this I have collected responses from users for three sets of same likert scale (5-point) based questionnaires after they tested each prototype.
Since the likert scale is ordinal, I have decided to apply Friedman Test to see if there is a statistically significant difference across all the prototypes and in case there is a significant difference, I will compare the prototypes pairwise by using a Wilcoxons Signed Rank test with a Bonferroni correction. What do you think about this approach?
Another area where I am confused is that I have 12 questions on the questionnaire set and these questions are divided amongst sub-group such as attractiveness, efficiency etc and I want to evaluate these sub-parameters across different prototypes. My question: what is the right approach of combining/aggregating values for questions belonging to a sub-group before applying the Friedman test?
Thank you for your time and for your response in advance!