A group of researchers is analyzing the effects of consuming caffeine (coffee or tea) before going to bed on sleep duration. Three groups of 25 people each were selected. In the first group, all participants consumed coffee within two hours before going to bed. In the second group, all participations consumed tea within two hours before going to bed, and participants from the third group were given no coffee or tea in the two hours before they went to bed. The file Sleep contains the sleep duration of each participant in minutes.

a. At the 0.05 level of significance, is there evidence of a difference in the mean sleep duration for the different groups?

b. At the 0.05 level of significance, is there evidence of a difference in the variation in the mean sleep duration of the different groups?

c. What effect does your result in (b) have on the validity of the results in (a)?

d. If appropriate, determine which groups differ in mean sleep duration.