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The visibility standard index (VSI) is a measure of Denver air pollution that is reported each day in the Denver Post. The index ranges from 0 (excellent air quality) to 200 (very bad air quality). During winter months, when air pollution is higher, the index has a mean of about 90 (rated as fair) with a standard deviation of approximately 30. Suppose that for 15 days, the following VSI measures were reported each day:
Make a control chart for the VSI, and plot the preceding data on the control chart. Identify all out-of-control signals (high or low) that you fi nd in the control chart by type (I, II, or III)? |
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