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The Four Possibilities for Any Process

Every unpredictable process is
subject to the effects of assignable causes.

by Donald J. Wheeler

Successful quality control requires making a clear distinction between product and process. Products may be characterized by conformance to specifications. Processes may be characterized by predictability. When combined, these two classification systems yield four possibilities for any process:

 Conforming and predictable -- the ideal state

 Nonconforming and predictable -- the threshold state

 Conforming yet unpredictable -- the brink of chaos

 Nonconforming and unpredictable -- the state of chaos


The ideal state occurs when a process is predictable and produces a 100-percent conforming product. Such predictability in a process results from using Shewhart's charts to identify assignable causes in  order to remove their effects. Product  conformity results from having natural process limits that fall within the specification limits.

How can a process achieve the ideal state? Only by satisfying four conditions:

 The process must remain inherently stable over time.

 The process must operate in a stable and consistent manner.

 The process average must be set at the proper level.

 The natural process spread must not exceed the product's specified tolerance.


Not satisfying any one of these conditions increases the risk of shipping a nonconforming product. When a process fulfills these four conditions, then a consistently conforming product results. The only way to determine that these four conditions apply to your process and subsequently are established and maintained day after day is by using Shewhart's charts.

The threshold state occurs when a    process is predictable but produces some nonconforming product. Sorting out nonconforming product is always imperfect and often very costly. The ultimate solution requires a change in either the process or the specifications.

If the nonconformity occurs because of an incorrectly set process average, then adjusting the process aim should help. Here Shewhart's charts can determine when to make adjustments. If the nonconformity occurs because the process's natural variation exceeds the specified tolerance, a reduction in the process variation may work. However, because a predictable process performs as consistently as possible, reducing the process variation will require a fundamental process change, which in turn will require evaluation.

As a final resort, the specifications themselves could change, with customer approval. Here, too, Shewhart's charts will prove invaluable. They are  essential not only in getting any process into the threshold state, but they also are critical in any attempt to move from the threshold to the ideal state.

     In the third state, the brink of chaos, processes are unpredictable even though they currently produce a 100-percent conforming product. While product conformity will lead to benign neglect, process unpredictability will result in periodic rude awakenings. The change from a 100-percent conforming product to some nonconforming product can come at any time and without the slightest warning.

Every unpredictable process is subject to the effects of assignable causes, the trouble source for any process. The only way to overcome the unpredictability of a process on the brink of chaos is to eliminate the effects of these assignable causes. This will require the use of Shewhart's charts.

The state of chaos exists when an unpredictable process produces some nonconforming product. The process's unpredictable nature will make some days look better than others but will also prevent effective elimination of the nonconforming product. Efforts to correct the problem ultimately will be foiled by the random process changes resulting from assignable causes.

Needed process modifications will produce only short-term successes because the assignable causes continue to change the process. With unnecessary modifications, a fortuitous shift by assignable causes may mislead. As a result, companies despair of ever operating the process rationally and begin to speak in terms of magic and art.

The only way to move a process out of chaos is to eliminate the effects of assignable causes. This requires the use of Shewhart's charts; no other approach will work consistently.

All processes belong to one of these four states, although processes may move from one state to another. In fact, entropy acts on every process, causing it to move toward deterioration and decay, wear and tear, breakdowns and failures. Because of entropy, every process will naturally and inevitably migrate toward the state of chaos. The only way to overcome this migration is by continually repairing entropy's effects.

Because processes in the state of chaos obviously require change, chaos managers inevitably are appointed to drag the process back to the brink of chaos, erroneously considered the "out-of-trouble" state in most operations. Once the process returns to the brink of chaos, then chaos managers leave to work on other problems. As soon as their backs are turned, the process begins to move back down the entropy slide toward chaos.

New technologies, process upgrades and other magic bullets can never overcome this cycle of despair. Technologies may change, but the benign neglect that inevitably occurs when the process teeters on the brink of chaos will allow entropy to drag the process back down to the state of chaos. Thus, focusing solely on conformance to specifications will condemn an organization to cycle forever between the two states.

Entropy places a process in the cycle of despair, and assignable causes doom it to stay there. Thus, it is important to identify both the effects of entropy and the presence of assignable causes. Shewhart's charts will consistently and reliably provide the necessary information in a clear and understandable form.

The traditional chaos-manager approach focuses on conformance to specifications but doesn't attempt to characterize or understand the behavior of a process. Therefore, about the best this approach can achieve is to get the process to operate on the brink of chaos some of the time.

Which explains why any process operated without Shewhart's charts is doomed to operate in the state of chaos.


About the author

Donald J. Wheeler is an internationally known consulting statistician and the author of Understanding Variation: The Key to Managing Chaos and Understanding Statistical Process Control, Second Edition.

New for 1998

This will be Donald Wheeler's last column for Quality Digest. Check out our new SPC columnist, Tom Pyzdek, debuting in our January issue.


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