In theory, there is no difference between theory and practice. In practice, there is. During the past three months we have looked at three families of probability models and found that they share some remarkable properties. These properties provide a theoretical explanation of how and why process behavior charts work as they do. However, the ultimate justification for Shewhart’s approach lies in the fact that it works in practice.

Two types of variation, two actions

When a process displays routine variation, we may think of the differences between items as being created by a constant system of a large number of “common causes” in which, according to Walter A. Shewhart, “no cause produces a predominating effect.” When this happens, according to W. Edwards Deming, “It will not be profitable to try to determine the cause of individual variations.” On the other hand, when a process displays exceptional variation, “it will be profitable to try to determine and remove the cause of the exceptional variation.”

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