Common Cause vs. Special Cause of Variation

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Common Causes vs. Special Causes of Variance

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It’s really not nice to always blame the humans. We deserve some of the credit, both good and bad, but not all. Have you heard of the source of variance being referred to as common cause vs. special cause?

Assign causes of variation in order to track error and be proactive.

When he developed the control charts, Dr. Shewhart recognized that certain variations were a part of every process and he accounted for them in his predictions of future events. He called these sources of variation random or chance causes and found there was no one event that caused them. Now called common causes, these variations are measured and documented in control charts.

 

It’s important to know the nuance of common causes vs. special causes of variance. Not all variation is built into the system. Some variations have identifiable causes such as human error, bad raw materials, or equipment failure. Known as assignable or special causes, these are the sources of variation that control charts are designed to signal.Assignable causes interfere with the process so it produces an irregular output. Control charts reflect this disturbance, allowing the problem to be tracked down. 

Zontec’s Synergy Suite of SPC software helps you identify variance. How? Schedule a Demo here.

 

This content is adapted from Zontec’s The Book of Statistical Process Control.
You can download a free copy here.

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