Variation and What It Means to be in Control and Capable

To make a thing the way we want to make it is one popular conception of control.

—Walter Shewhart

Why is it that there is so much variability in what we do? Why can’t we better predict how our processes will perform from day-to-day or even hour-to-hour? If you find yourself asking questions such as these, statistical process control (SPC) is a tool you will find useful. Let us begin with a discussion of variation.


More than 70 years ago, Walter Shewhart, the father of SPC, asked, “What can we say about the future behavior of a phenomenon acting under the influence of unknown or chance causes? I doubt that, in general, we can say anything.”1 Shewhart went on to discuss ...

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