IE 4453 Chapter : Ch05
Document Summary
Chance or common causes of variability represent the inherent, natural variability of a process - its background noise. Variation resulting from assignable or special causes represents generally large, unsatisfactory disturbances to the usual process performance. Assignable cause variation can usually be traced, perhaps to a change in material, equipment, or operator method. A shewhart control chart can be used to monitor a process and to identify occurrences of assignable causes. There is a high probability that an assignable cause has occurred when a plot point is outside the chart"s control limits. By promptly identifying these occurrences and acting to permanently remove their causes from the process, we can reduce process variability in the long run. The control chart is mathematically equivalent to a series of statistical hypothesis tests. If a plot point is within control limits, say for the average x , the null hypothesis that the mean is some value is not rejected.
