Acidity of rainwater. The Normal quantile plot in Figure 3.32 (page 143) shows that the acidity (pH) measurements for rainwater samples in Exercise 3.38 are approximately Normal. How well do these scores satisfy the 68-95-99.7 rule? To find out, calculate the mean and standard deviation s of the observations. Then calculate the percent of the 105 measurements that fall between - s and + s and compare your result with 68%. Do the same for the intervals covering two and three standard deviations on either side of the mean. (The 68-95-99.7 rule is exact for any theoretical Normal distribution. It will hold only approximately for actual data.)