Shewhart's Charts

Any program designed to increase organizational

efficiency that does not use SPC is doomed to fail.

Last month, I described four myths relating to Shewhart's charts. This month I will discuss four foundations of the charts.

Three-sigma limits are action limits-they dictate when action can be taken on a process. They are not probability limits. While they have a basis in probability theory, three-sigma limits were chosen because they provided reasonable action limits. They strike an economical balance between the two possible errors you can make in interpreting data from a continuing process. Three-sigma limits neither result in too many false alarms nor do they miss too many signals. In addition, they are unaffected by data nonnormality, even when the subgroup size is one.

The choice of dispersion statistic is unimportant-ranges, standard deviations or root mean square deviations may be used. If the proper approach is used, different statistics will yield similar results. If the wrong approach is used, different statistics will yield similar incorrect results.

Failure to consider these factors when placing data on a control chart can result in nonsensical control charts. The effective use of Shewhart's charts requires an understanding of rational sampling and rational subgrouping.

This is why so many of W. Edwards Deming's 14 points bear directly upon this one foundation and why SPC alone isn't enough. On the other hand, any program designed to increase organizational effectiveness and efficiency that does not use SPC is doomed to fail.

This fourth foundation of Shewhart's charts is only implicit in Shewhart's work-there was always the assumption that organizations behave in a rational manner. However, Deming came to see that this wasn't the case. Simply giving people effective methods for collecting, organizing and analyzing data wasn't enough. In the absence of such methods, businesses had come to be run by the emotional interpretation of the visible figures-a universal "My mind is made up, don't bother me with the facts" syndrome.

While Deming's 14 points do not constitute the whole of his philosophy, they are a profound starting point. They are not a set of techniques, a list of instructions nor a checklist. They are a vision of just what can be involved in using SPC. And they ultimately lead to radically different and improved ways of organizing businesses and working with people. However, a deep understanding is required before these 14 points can be used to accomplish the total transformation. The need is not to adopt the 14 points individually or collectively, but rather to create a new environment conducive to their principles.

For further reading about Deming's 14 Points, I recommend The Deming Dimension by Dr. Henry Neave (SPC Press).

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

© 1996 SPC Press Inc. Telephone (423) 584-5005.