15. The file tumor.dat contains data on the number of days to occurrence of a mammary tumor in 48 rats injected with a carcinogen and subsequently randomized to receive either the treatment or the control. The data, collected by Gail et al. (1980), represent only a portion of the results they reported. (Here, the data are the number of days to the first tumor. Most rats developed multiple tumors, but these results are not considered here.) Compare the means of the two groups with Welch’s method for means, and verify that you reject with α = .05. Examine a boxplot, and comment on what this suggests about the accuracy of the confidence interval for µ1 − µ2. Verify that you also reject when comparing Mmeasures of location. What happens when comparing 20% trimmed means or when using the Kolmogorov–Smirnov test?