Assignable Causes - Causes of Variability

Variability in process output may also be the result of external causes which are separately identifiable, and which are not inherent to the process. Examples of these might include incorrectly setting the process, tooling wear, equipment wear or a change of operator. 

When a process is subject only to its common causes of variability it is deemed to be stable, the variability of its outputs are predictable within statistical limits, and it is considered to be in control. Process control may then be seen as the detection and eradication of variability generated by assignable causes: SPC provides tools to help in this task.