Common causes refer to occurrences that contribute to the natural variation in any process. Special causes are unusual occurrences that are not normally (or intentionally) part of the process that unsettle its stability. While some degree of common cause variation will naturally occur in any process, it's important to identify and attempt to eliminate special causes of variation. Here are some examples.
|
Process |
Common Cause of Variation |
Special Cause of Variation |
|
Baking a loaf of bread |
The oven's thermostat allows the temperature to drift up and down slightly. |
Changing the oven's temperature or repeatedly opening the oven door during baking can cause the temperature to fluctuate needlessly. |
|
Recording customer contact information |
An experienced operator makes an occasional error. |
An untrained operator new to the job makes numerous data entry errors. |
|
Injection molding of plastic toys |
Slight variations in the plastic from a supplier result in minor variations in product strength from batch to batch. |
Changing to a less reliable plastic supplier leads to an immediate shift in the strength and consistency of your final product. |
While it's important to avoid special cause variation, trying to eliminate common cause variation can make matters worse. Consider the bread baking process described above. Part of the natural variation of the process is due to the slack in the oven's thermostat. If you tried to correct this variation by manually turning the temperature setting up and down, you'd likely succeed only in adding more variability to the process. This is called overcorrection.
Control charts are useful to identify the occurrence of special cause variation.