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Common Cause Variation

Common cause variation is the predictable, repetitive, systemic portion of variation. Contrast this with special cause variation, caused by unusual occurrences.

Common cause variation, in a nutshell, is the consistent randomness built into a process. It is also frequently referred to as ‘noise.’

While common cause and special cause can be layman distinctions, they are also mathematically calculable. Control charts are the best example of this.

Control charts look at the historical performance of a process and create control limits at a specified standard deviation from the average. The fluctuations within that defined range are typically common cause variation. Those that exceed that range are attributed to special causes.

Misconceptions about Common Cause Variation

There are a few misconceptions about common cause variation.

The first is that it is not as big of a deal as special cause variation. While special causes typically cause a bigger blip on performance charts, common cause variation is ever-present, relentlessly sapping productivity and quality. And just because the variation is consistent, it can be quite large. The spread within the expected, historical ranges can be quite large. Bottom line: both common cause and special cause can have a tremendous impact.

The second is that common cause variation is unexplained or random. That is grossly inaccurate. Think of an icicle melting from a gutter and dripping into the snow below. There is a pattern that forms. A small cluster is around the middle—the common cause variation. There will also be a fair number of outlying drips. Those are the special causes. They may be caused by a door slamming, a gust of wind, or a bird landing on the gutter. But the pattern in the middle, though seemingly random, is also explainable. It could be because of the vibration of a heater duct running inside the roof. It could be from the pattern of footsteps up and down a hallway throughout the day. It could be from the vibration of a nearby highway. The point is that common cause, though random (or random appearing) is often explainable with a small enough microscope. Don’t simply dismiss common cause variation.


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