The level of dissolved oxygen in water was measured every 2 hours in a river where industrial plants discharge processed waste. Each observation consists of four samples, from which the sample mean and range of the amount of dissolved oxygen in parts per million are calculated. Given Table shows the results of 25 such observations.
Discuss the stability of the amount of dissolved oxygen. Revise the control limits, if necessary, assuming special causes for the out-of-control points. Suppose that environmental standards call for a minimum of 4 ppm of dissolved oxygen. Are these standards being achieved? Discuss.
