Statistical process control

Statistical process control uses statistical methods to monitor process variation, identify changes, and distinguish stable performance from unusual variation.

Control chart: A time-ordered graph with statistically derived limits used to detect changes in a process. The central SPC tool for separating routine variation from signals of change.
Process variation: The differences among outcomes produced by a process over time or across units. SPC begins by measuring and interpreting this variation.
Walter A. Shewhart: An American physicist and statistician who developed control charts and foundational ideas in statistical quality control. His 1920s work established the chart-based approach at the heart of SPC.
Acceptance sampling: A quality-control method that inspects a sample to decide whether to accept or reject a lot. Unlike SPC, it makes decisions about lots rather than continuously monitoring process behavior.
Six Sigma: A quality-management approach that uses data and structured projects to reduce defects and process variation. Six Sigma projects may use SPC, but its scope extends beyond ongoing process monitoring.
Shewhart chart: A control chart that detects relatively large shifts by comparing plotted statistics with control limits. It provides the classic SPC approach to identifying abrupt process changes.
Statistical hypothesis testing: A framework for evaluating evidence in sample data against a stated statistical model or hypothesis. Control-chart signals are statistical evidence, not proof that a cause has been found.
Bell System: The American telecommunications system of companies that operated under the Bell name for much of the twentieth century. Shewhart developed statistical quality methods while working at Bell Laboratories, part of this system.
Individuals control chart: A control chart for measurements collected one at a time, often paired with a moving-range chart. It applies SPC when rational subgroups of multiple observations are unavailable.
Taguchi methods: Design and analysis methods developed by Genichi Taguchi to improve product and process robustness. They emphasize designing quality into systems, complementing SPC’s monitoring of operating performance.

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