Rojas, Sherrit, Harris, and Knight (2008) conducted a study using the Car, Relax, Alone, Forget, Friends, Trouble (CRAFFT) substance abuse screening test in 14- to 18-year-old patients. This screening test is a 16-item test for substance-related problems and disorders among adolescents. The researchers screened patients and compared the scores in their study (Study 2) to those from a previous study (Study 1). They reported, because the CRAFFT score distributions were highly skewed, we used the nonparametric Mann-Whitney U test for differences in mean rank.

The mean rank in Study 1 was significantly higher than in Study 2 (mean rank 362 vs. 325, p = .02). (Rojas et al., 2008, p. 195)

(a) Why was the Mann-Whitney U used in this study?

(b) Based on the description of the results, did Study 1 or Study 2 generate the test statistic? Explain.