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statistical process control (SPC)
Process control, or SPC techniques use statistical theory to determine if a process is in a state of statistical control.
Statistical control is defined by the absence of assignable cause variation.
When a process is in this state the only variation is caused by naturally occurring or normal cause variation.
The following table summarizes the difference between the two causes of variation.
| Characteristic | Assignable | Normal |
| alternative name | special | common |
| presence | sporadic | always present |
| cumulative impact | large | small |

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