# Statistical Process Control (SPC)

Operations · `SPC` · verification: source checked

Statistical Process Control (SPC) is a method for monitoring a process by comparing current measurements with control limits calculated from historical data. Most measurements should fall within the control limits; points outside the limits are investigated to see whether they belong to the same population as the initial snapshot or model. SPC uses tools such as histograms, check sheets, Pareto charts, cause and effect diagrams, defect concentration diagrams, scatter diagrams, and control charts.

## Specification

| Property | Value |
|---|---|
| Kind | methodology |

## Study notes

- SPC compares what is happening today with what happened previously.
- Phase I uses historical data to compute initial control limits; points outside the limits are investigated and possibly discarded, then the limits are recomputed.
- Phase II is real-time process monitoring using the limits from the end of Phase I.
- Typical SPC tools include histograms, check sheets, Pareto charts, cause and effect diagrams, defect concentration diagrams, scatter diagrams, and control charts.
- Process control is the active changing of the process based on the results of process monitoring.
- Out-of-control action plans (OCAPS) detail the action to be taken once an out-of-control situation is detected.
- Advanced process control loops are automated changes to the process programmed to correct for the size of the out-of-control measurement.

## Sources

1. [6.1.2. What are Process Control Techniques?](https://www.itl.nist.gov/div898/handbook/pmc/section1/pmc12.htm) (accessed 2026-10-25)
2. [6.1.3. What is Process Control?](https://www.itl.nist.gov/div898/handbook/pmc/section1/pmc13.htm) (accessed 2026-10-25)

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