What are chance and assignable causes of variability? What part do they play in
ID: 3126089 • Letter: W
Question
What are chance and assignable causes of variability? What part do they play in the operation and interpretation of a Stewhart control chart? What is meant by the statement that a process is in a state of statistical control? If a process is in a state of statistical control, docs it necessarily follow that all or nearly all of the units of the product produced will be within specification limitations? What is underlying logic behind the use of three sigma limits on Stewhart control charts? How will the chart behave if narrower limits are chosen? How will it respond if wider limits are chosen?Explanation / Answer
9. “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.
10.A process is said to be in statistical control if the response variable falls within +/- 3 std. deviations of mean.
11. A process is in a state of statistical control does not mean that all the units of produced will be within the specification limits