Shewart:
- Chance-cause: Implies that the variation is random and is inherent to the design of the system. However this is internal variation that is controllable by modifying policy and/or changing process.
- Assignable-cause: Implies that there is an easy cause-and-effect relationship between an external event and the variation. Cannot be controlled by local management.
Alpert / Deming
- Common-cause: Implies that the variation is common to all similarly designed systems so it’s nothing special. Probabilistically predictable but otherwise noise within the system.
- Special-cause: Signals that something happened that was a surprise (i.e., new knowledge or an event that is different to how the system normally works). Unpredictable.
Feigenbaum
- Usual: The amount of variation that you’ve learned to expect of the system.
- Unusual: Any variation that is greater than what you’d expect.
I kind of like the simplicity of Feigenbaum’s naming but it’s useful to know about all the variants to have a better understanding of what they’re all trying to convey.