ACTIVITY

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 some particular characteristics of the product. The cost of desctructive 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 the following table.

a. Form a 95% confidence interval for the mean number of characters printed before the printhead fails.

• Step 1: Make sure that "Input" menu option is selected.
• Step 2: Click on "Col_1" and replace it with "Number".
• Step 3: Enter the data.
• Step 4: Select the "t test" from menu options.

The output will include: Sample size, Mean, and Standard deviation followed by 95% confidence interval for the population mean.

b. Find a 99% confidence interval by following the steps given below:

• Step 1: Make sure that "Input" menu option is selected.
• Step 2: Click on "Col_1" and replace it with "Number".
• Step 3: Enter the data.
• Step 4: Select the "t test" from menu options
• Step 5: Click on the "Options" button on the upper right hand side of the window.
• Step 6: Change only Alpha from 5.0 to 1.0.
• Step 7: Click on the "OK" button.

The output will include: Sample size, Mean, and Standard deviation followed by 95% confidence interval for the population mean.

c. Compare the length of the confidence intervals that you have found in a and b. Which one is shorter?