Refer to the data for problem 11. Continue using the logarithms of the lengths of calls.
a. Find the average difference and its standard error for every pair of types of calls (subtracting smaller from larger in each case).
b. Which pairs of types of calls are significantly different from each other, in terms of average logarithm of length?

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TABLE 15.5.3 Lengths of Telephone Calls Information Sales Service Other 6.3 1.2 0.6 5.1 5.2 2.9 2.6 1.2 7.0 14.2 8.4 0.6 4.4 26.6 7.4 2.5 3.0 2.6 0.8 3.8 0.4 0.6 2.2 12.3 4.2 2.8 7.0 3.9 26.7 1.2 7.7 173 7.8 4.3 3.4 1.3 2.0 7.2 2.7 3.4 13.3