Penner and Watts (1991) investigated whether the time to drill holes in rock differs using "dry" or "wet" drilling. In dry drilling, compressed air is used to flush the cuttings, and in wet drilling water is used. Each method is used in 12 rocks. The drilling times (in 1/100 minutes)
Wet Dry
607 727
549 965
762 904
665 987
588 847
798 918
704 814
772 750
780 804
599 989
603 902
699 939
a. Analyze these data using parallel boxplots.
b. Conduct the appropriate hypothesis test using 0.01 significance level.
c. Construct a 99% confidence interval for the true difference in the mean time
d. What did you assume to do the analysis? Can you evaluate how well these data meet the assumptions? If yes, determine how comfortable you are with them. If not, explain why.
e. Discuss the conclusions of the hypothesis test and confidence interval. Which method would you recommend?