As defined in the statistical thinking and methods of Walter A. Shewhart and W. Edwards Deming., Common and special causes are the two distinct origins of variation in a process:
- common causes, also called Natural patterns, are the usual, historical, quantifiable variation in a system,
- while special causes are unusual, not previously observed, non-quantifiable variation.
Shewhart conclude that while every process displays variation:
- some processes display controlled variation that is natural to the process,
- while others display uncontrolled variation that is not present in the process causal system at all times
- wiki/natural_pattern - natural pattern