Manufacturers love to hear the word “in control” when talking about processes. From a statistical process control (SPC) point of view, an in-control process means that a process is stable or predictable. After putting in the work to get a process controlled, how do you make sure it stays in control?

Every process has variation. Although some sources of variation may be known and considered minor, others, if deemed critical, must be detected and removed to maintain a stable process.

Walter A. Shewhart and W. Edwards Deming identified the following two sources of process variation:
Common cause. Variation that is inherent as part of the process. Examples of common cause are natural wear and tear, changes in humidity, and aging machinery.
Special cause. Variation that is outside of the normal process. Examples of special cause are operator error, broken parts, and power outages.

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