136 Estimating the age of glacial drifts. Tills are glacial drifts consisting of a mixture of clay, sand, gravel, and boulders.

Engineers from the University of Washington’s Department of Earth and Space Sciences studied the chemical makeup of buried tills in order to estimate the age of the glacial drifts in Wisconsin ( American Journal of Science , Jan. 2005). The ratio of the elements aluminum

(AI) and beryllium (Be) in sediment is related to the duration of burial. The Al–Be ratios for a sample of 26 buried till specimens are given in the accompanying table and saved in the TILLRATIO file. An SPSS printout with descriptive statistics for the Al–Be ratio is shown at the bottom of the page.

a. Find and interpret the z-score associated with the largest ratio, the smallest ratio, and the mean ratio.

b. Would you consider the largest ratio to be unusually large? Why or why not?

c. Construct a box plot for the data and identify any outliers.

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3.75 4.05 3.81 3.23 3.13 3.30 3.21 3.32 4.09 3.90 5.06 3.85 3.88 4.06 4.56 3.60 3.27 4.09 3.38 3.37 2.73 2.95 2.25 2.73 2.55 3.06 Based on American Journal of Science, Vol. 305, No. 1, Jan. 2005, p. 16 (Table 2).