Assume the following research experience. A group of researchers went to a school. On the first day, they gave a questionnaire about a topic for the students to fill. The next day, the researchers gave the same students a lecture about that topic. The study finished on the 3rd day when the researchers gave the same questionnaire and collected the answers from the same students.

The questionnaire collects age groups, gender, and other descriptive categorical variables. The scale of the questions about the topic was the Likert-type. Also, data are not paired.

The group of researchers now wants to compare, e.g., age groups, to conclude how effective the intervention was. For example, for a given question, did age group 15–18 reveal differences before and after intervention? To accomplish this, we are doing Mann-Whitney U-tests on every subsample of interest.

Is this the correct approach? Should the p-value be corrected (e.g., Bonferroni correction)?