You can take one of two approaches to controlling the quality of your product. Once manufactured, you can compare it against its specifications and sort it as either conforming or nonconforming. This, however, will guarantee the greatest degree of variability between units as anything within specifications is considered acceptable. Alternatively, you can work to run the manufacturing process as consistently as possible to produce units that vary as little as possible from one another.
There is no path to improving performance with the first approach where you sort manufactured product as conforming or nonconforming. However, there is a clear path to improving performance through the use of process control techniques introduced by Dr. Walter Shewhart. But, how do you gauge improvement? One measure is achieving a state of statistical control for the process so that its behavior is predictable. Another measure is the manufacture of 100 percent conforming product by the process.
When these two measures are taken together, there are four possible states a process can be in (see figure below): The process is
- predictable and producing conforming product – the ideal state
- predictable but producing nonconforming product – the threshold state
- not predictable but producing conforming product – the brink of chaos
- not predictable and producing nonconforming product – the state of chaos
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.
- 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