Prior to the Industrial Revolution, manufacturing consisted of making things by hand. Each part was custom-made to fit in with the other parts in each assembly, with the result that every product was unique and expensive.
As early as 1793, Eli Whitney had the idea of the interchangeability of parts. While this idea was revolutionary, it was also difficult to implement. The problem was how to make the parts interchangeable. Try as one might, the parts would not turn out to be identical. Therefore, manufacturers had to be content with making them similar. Specifications were developed to define how similar the parts had to be in order to fit, and all variation was classified as either permissible (within the specifications) or excessive (outside the specifications).
The specification was a guide for defining the difference between a "good" part and a "bad" part. But it did not tell manufacturers how to make good parts, nor did it help them discover why bad parts were being produced. All they could do with specifications is sort the good stuff from the bad stuff at the end of the production line. Thus, manufacturing became an endless cycle of fabrication, inspection and rework, with some good product escaping every now and then.
Of course, the customer needed more good product than was leaking out of the manufacturing process, and so the manufacturers began to write deviations from the specifications in order to get more good stuff to ship. And this is the origin of the perpetual argument about how good the parts must be. Manufacturers seek relaxed specifications, customers demand tighter specifications, and the engineers are caught in the middle.
This conflict obscured the original and fundamental issue -- how to manufacture parts with as little variation as possible. The original ideal had been to make parts that were essentially identical. But how can we do this? A state of virtually uniform product can be achieved only through the careful study of the sources of variation in the process, and through action by management to reduce -- or to eliminate entirely -- sources of extraneous variation. Shewhart's charts provide a way to do just this.
Shewhart's control charts allow you to characterize a given process as being predictable or unpredictable. A predictable process operates as consistently as possible; an unpredictable process does not. And this distinction is the beginning of the journey of continual improvement.
When your process is unpredictable, it will display excessive variation that can be attributed to assignable causes. By the very way the charts are set up, it will be worthwhile to look for any assignable cause of unpredictable process changes. As the charts guide you to those points in space and time that are connected with the unpredictable process changes, they help you discover ways to improve your process, often with little or no capital expense.
On the other hand, when your process is predictable, it will be a waste of time to look for assignable causes of excessive variation. There is no evidence of the presence of assignable causes, and looking for such will simply be a waste of time and effort. When a process is already operating as consistently as possible, the only way to improve it will be to change it in some fundamental manner.
A predictable process operates up to its potential; an unpredictable process does not. Are you getting the most out of your process?
Shewhart's charts give you the means of identifying the voice of the process. This is distinctly different from specifications, which are, at best, the voice of the customer. Thus, we need to distinguish between three different types of action:
Specifications are for taking action on the product -- to separate the good stuff from the bad stuff after the fact.
Shewhart's charts are for taking action on the process -- to look for assignable causes when they are present, with an eye toward process improvement, and to refrain from looking for assignable causes when they are absent.
Actions to align the two voices are desirable -- While this has been tried in the past, the lack of a well-defined voice of the process has made alignment difficult to achieve.
These three types of action have different objectives. All are valid, all are reasonable, but the first and the third are strictly concerned with maintaining the status quo. Shewhart's charts are the one tool that will facilitate the continual improvement of both process and product.
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. He may be reached via e-mail at firstname.lastname@example.org.