The visibility standard index (VSI) varies from 0 (excellent) to 200 (very bad). Denver's mean VSI is 90 with a standard deviation of 30. We record the following data:
Day: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
VSI: 30 15 45 55 60 88 99 95 91 110 165 160 120 140 181
Use a control chart to determine what signals, if any, indicate that the process is out of control. Then, list the days the signal occurred and the type of signal (I, II, or III).