Common and Special Cause Variation
Common cause variation is the natural, built-in variation of a process: many small causes acting all the time, giving a stable and predictable pattern. Special cause variation (also called assignable cause) comes from a specific, identifiable event that is not part of the normal process, such as a broken tool, a bad batch of material, or a new operator. Control charts are designed to tell the two apart, because they need opposite responses: special causes are found and removed locally, while common-cause variation only goes down when the system itself is changed, which usually needs management action.
Common versus special cause
| Common cause | Special cause | |
|---|---|---|
| Other names | Chance, random, natural, general | Assignable |
| Source | Many small factors that are always present | A specific event or change |
| On a control chart | Random points inside the control limits | Points outside the limits or non-random patterns |
| Who fixes it | Management, by changing the system (for example, upgrading a machine) | Usually the people closest to the process (for example, fixing a machine fault or rejecting a bad lot) |
| Examples | Normal wear, small temperature swings, normal differences between trained operators | A broken tool, a wrong setting, a new untrained operator, a bad raw-material lot |
The cost of mixing them up
Treating common-cause noise as if it were special — adjusting the process after every result — is called tampering, and it makes variation worse, not better. W. Edwards Deming demonstrated this with his funnel experiment. The opposite mistake is ignoring a real special cause. The control chart, first developed by Walter Shewhart, gives a rule for when to act and when to leave the process alone.
Frequently asked questions
Is common cause variation always acceptable?
No. It is predictable, but it can still be too large for the customer’s specification. Reducing it is a typical Six Sigma project, and it needs changes to the process itself.
Who named common and special causes?
Walter Shewhart used the terms chance causes and assignable causes. W. Edwards Deming later popularized the names common causes and special causes.
See also: Process capability · Multi-vari chart: three families of variation
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