ABSTRACT
All too often processes are adjusted on the basis of a single result or measurement (n = 1), a practice which can increase the apparent variability. As pointed out in Chapter 4, a control chart is like a traffic signal, the operation of which is based on evidence from process samples taken at random intervals. A green light is given when the process should be allowed to run without adjustment, only random or common causes of variation being present. The equivalent of an amber light appears when trouble is possible. The red light shows that there is practically no doubt that assignable or special causes of variation have been introduced; the process has wandered.
