In “What State Is Your Process In?” I noted that a process can be in one of four possible states. Here I write about the Ideal State wherein a process is predictable and is producing 100 percent conforming product.
A process that is predictable is one that is in a state of statistical control. The variability of the product from one unit to the next is randomly distributed about the average and bounded within statistically established limits – its natural limits (red solid lines in the figure below). So long as the process remains “in control”, it will continue to produce units within these limits.
Complete product conformity comes about when the process’s natural limits fall within the product’s specification limits (blue solid lines in the figure below). This depicts the ideal state for a process.
In order for a process to achieve this ideal state
- The process must be inherently stable over time. This means that in the absence of external disturbances – what Dr. Shewhart referred to as assignable causes – the process’s natural variability does not change over time. (Note: There are processes that are inherently chaotic. An excellent reference to such processes is “Nonlinear Dynamics And Chaos” by Professor Steven Strogatz)
- The process must be operated in a stable and consistent manner. The operating conditions cannot be selected or changed arbitrarily. Often machine parameters are tweaked in response to natural fluctuations in the process’s output. These actions add to the process’s natural variation disrupting its stability. Dr. Deming demonstrated the effects of such tampering through the “Nelson Funnel Experiment”. (Bill Scherkenbach has an excellent discussion of it in “Deming’s Road to Continual Improvement”.)
- The process average must be set and maintained at an appropriate level. If you refer to the charts above, you can imagine the consequence of moving the process average up or down from its aim. The result is the production of nonconforming product either on the high or low side.
- The natural tolerance of the process must be less than the specified tolerance for the product. This is obvious upon a first glance at the second chart above.
A process that satisfies these four conditions will be in the ideal state and the manufacturer can be confident that he is shipping only conforming product. In order to maintain the process in the ideal state he must use process control charts. He must act on the signals they provide to promptly identify assignable causes and eliminate their effects.
Note: I learned this material from reading Dr. Wheeler’s writings. My post is intended to reflect my current understanding. None of the ideas herein are original to me. Any errors are my failures alone.
- Moen, Ronald D., Thomas W. Nolan, Lloyd P. Provost. Improving Quality Through Planned Experimentation. McGraw-Hill. 1991. Print. ISBN 0-07-042673-2
- Scherkenbach, William W. Deming’s Road to Continual Improvement. Knoxville, TN: SPC Press, Inc. 1991. Print. ISBN 0-945320-10-8
- Strogatz, Steven H. Nonlinear Dynamics And Chaos: With Applications To Physics, Biology, Chemistry, And Engineering. Reading, MA: Addison-Wesley. 1995. Print. ISBN 0-201-54344-3
- Wheeler, Donald J. and David S. Chambers. Understanding Statistical Process Control. Knoxville, TN: SPC Press, Inc. 1986. Print. ISBN 0-945320-01-9
- Wheeler, Donald J. The Four Possibilities for Any Process. Quality Digest. 1997. Web. http://www.qualitydigest.com/dec97/html/spctool.html