The Pursuit of Excellence
A Manager's Guide to Quality 		Reaching New Heights


By employing what are known as the Western Electric Zone Tests (so named because they were first published in the Western Electric Corporation's Statistical Quality Control Handbook in 1956), it is possible to test whether the adjustment has been successful. The zone tests are given in the form of occurrences which indicate a lack of control.

TEST 1: A single point falling beyond the 3 sigma control limits.
TEST 2: Two out three successive points more than 2 sigma units away from the centre line (on the same side).
TEST 3: Four out of five successive points more than one sigma unit away from the centre line (on the same side).
TEST 4: Eight successive points on the same side of the centre line.

If any one of these situations occurs, it is likely that the process aim has not been set correctly. In such a case, a further adjustment is warranted. A good guide for establishing the amount of adjustment required is to calculate the average of all the values collected up to the point when one of the tests resulted in a positive signal. This process should be repeated until ten successive values have all tested negative. At this point it is reasonable to assume that the adjustment has had the desired effect, and the process can continue to be monitored with a normal mean and range chart.

This process is demonstrated in Exercise 7 on the bead board, which you may perform now by going back to your bead board window (if you used it ealier) or by opening it from the Resources page for which there is a link in the shaded area in the header.