Some quality control experiments require destructive sampling (i.e., the test to determine whether the item is defective destroys the item) in order to measure a particular characteristic of the product. The cost of destructive sampling often dictates small samples. Suppose a manufacturer of printers for personal computers wishes to estimate the mean number of characters printed before the printhead fails.The printer manufacturer tests n = 15 printheads and records the number of characters printed until failure for each. These 15 measurements (in millions of characters) are listed in Table 5 , followed by a MINITAB summary statistics printout in Figure 10 .

a. Form a 99% confidence interval for the mean number of characters printed before the printhead fails. Interpret the result.

b. What assumption is required for the interval you found in part a to be valid? Is that assumption reasonably satisfied?

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Table 5 Number of Characters (in Millions) for n = 15 Printhead Tests 1.13 1.55 1.43 .92 1.25 1.36 1.32 .85 1.07 1.48 1.20 1.33 1.18 1.22 1.29 Data Set: PRINTHEAD Variable N Mean NUMCHAR 15 1.23867 0.19316 StDev SE Mean 0.04987 99% CI (1.09020, 1.38714)