In the on-line paper "Changes in Lake Ice: Ecosystem Response to Global Change" (Teaching Issues and Experiments in Ecology,, 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?

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