Gender, aggression, and the interpretation of a Mann–Whitney U test: Spanish researchers examining aggression in children’s dreams reported the following: “Using the Mann–Whitney nonparametrical statistical test on the gender differences, we found a significant difference between boys and girls in Group 1 for overall [aggression] (U = 44.00, p = 0.004) and received aggression (U = 48.00, p = 0.005). So, in their dreams, younger boys not only had a higher level of general aggression but also received more severe aggressive acts than girls of the same age” (emphasis in original) (Oberst, Charles, & Chamarro, 2005, p. 175). 

a. What is the independent variable, and what are its levels? What is the dependent variable? 

b. Is this a between-groups or within-groups design? 

c. Which hypothesis test did the researchers conduct? Why might they have chosen a nonparametric test? Why do you think they chose this particular nonparametric test? 

d. In your own words, describe what they found. 

e. Can we conclude that gender caused a difference in levels of aggression in dreams? Explain. Provide at least two reasons why gender might not cause certain levels of aggression in dreams even though these variables are associated.