Ecosystem Response. In the on-line paper “Changes in Lake Ice: Ecosystem Response to Global Change” (Teaching Issues and Experiments in Ecology, tiee.ecoed.net, Vol. 3), R. Bohanan et al.

questioned whether there is evidence for global warming in long-term data on changes in dates of ice cover in three Wisconsin Lakes. The following table gives data, for a sample of eight years, on the number of days that ice stayed on two lakes in Madison, Wisconsin—Lake Mendota and Lake Monona.
Year Mendota Monona 1 119 107 2 115 108 3 53 52 4 108 108 5 74 85 6 47 47 7 102 96 8 87 91

a. Obtain a normal probability plot and boxplot of the paired differences.

b. Based on your results from part (a), is performing a paired t-test on the data reasonable? Explain your answer.

c. At the 10% significance level, do the data provide sufficient evidence to conclude that a difference exists in the mean length of time that ice stays on these two lakes?