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.

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?

YearMMendota Monona 119 115 53 108 74 47 102 87 107 108 52 108 85 47 96 91 4 6