Control Chart: Air Pollution 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:

Day 1 2 3 4 5 6 7 8 VSI 80 115 100 90 15 10 53 75 Day 9 10 11 12 13 14 15 VSI 80 110 165 160 120 140 195 Make a control chart for the VSI, and plot the preceding data on the control chart. Identify all outof-

control signals (high or low) that you find in the control chart by type (I, II, or III).

AppendixLO1