80. Windy. Alternative sources of energy are of increasing interest throughout the energy industry. Wind energy has great potential. But appropriate sites must be found for the turbines. To select the site for an electricity-generating wind turbine, wind speeds were recorded at several potential sites every 6 hours for a year. Two sites not far from each other looked good. Each had a mean wind speed high enough to qualify, but we should choose the site with a higher average daily wind speed. Because the sites are near each other and the wind speeds were recorded at the same times, we should view the speeds as paired. Here are the summaries of the speeds (in miles per hour):

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Variable Count Mean StdDev site2 1114 7.452 3.586 site4 1114 7.248 3.421 site2 - site4 1114 0.204 2.551 Is there a mistake in this output? Why doesn't the Pythag- orean Theorem of Statistics work here? In other words, shouldn't SD(site2 - site4) = VSD (site2) + SD (site4)? But V(3.586)2 + (3.421) = 4.956, not 2.551 as given by the software. Explain why this happened.