Glossary · Control theory and process control
Statistical process control (SPC)
Also known as: SPC, Control charting
German: Statistische Prozessregelung (SPC)
In quality management, statistical process control (SPC) is the use of statistical methods, in particular control charts, to monitor a process and distinguish common-cause variation from special-cause variation that calls for investigation.
- Control theory
- Standards
In one sentence
SPC uses statistical methods such as control charts to monitor process variation and detect special causes that need investigation.
Example
An injection molding line plots the part weight of five parts per hour on an X-bar and R chart; a run of points above the center line triggers a check of the barrel temperature.
How it applies
- Engineering: SPC data come from measurements taken in the process or at inspection stations, often stored in a Historian or a quality system. Control limits are calculated from process data, not from specification limits.
- Operation: Out-of-control signals lead to investigation and corrective action, for example through Corrective and preventive action (CAPA); they don't automatically adjust the process.
- Documentation: Document which characteristics are monitored, sample size and frequency, chart type, rules for out-of-control signals and the reaction plan. Operator instructions should state exactly what to do when a chart signals.
SPC vs. process control
Despite the name, SPC is monitoring, not automatic control. Process control adjusts the process continuously through control loops. The German term "statistische Prozessregelung" invites confusion; "statistische Prozesslenkung" is also used.