Statistical Process Control (SPC)

Statistical process control is the use of statistical methods, chiefly control charts, to monitor a process over time, distinguish natural variation from signals of change, and keep the process stable and predictable.

SPC rests on the idea that every process varies. Walter Shewhart, working at Bell Telephone Laboratories in the 1920s, distinguished common-cause variation, inherent in a stable process, from special-cause variation, arising from specific identifiable events. Samples of a quality characteristic, such as a dimension, weight or temperature, are plotted on control charts with limits calculated from the process's own data, typically three standard deviations either side of the centre line. Points outside the limits or non-random patterns signal that a special cause should be investigated.

SPC is used in machining, moulding, chemical processing, electronics assembly and many other industries to detect problems before they produce non-conforming product, to avoid over-adjustment and to give customers evidence of process stability. Automotive suppliers apply it as one of the core quality tools, and it is often specified in control plans. Once a process is stable, capability indices such as Cp and Cpk compare its variation with the specification limits.

Control limits describe what the process does, while specification limits describe what the customer requires, and the two must not be confused. Effective SPC needs rational subgrouping, an adequate measurement system and clear reaction plans when a signal appears. Many plants now collect SPC data automatically from gauges and machines. The ISO 7870 series provides guidance on control charts.

Key points

Uses control charts to separate common-cause from special-cause variation.
Founded on Walter Shewhart's work at Bell Telephone Laboratories.
Control limits are usually set three standard deviations from the centre line.
Control limits come from process data; specification limits come from requirements.
The ISO 7870 series provides guidance on control charts.

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Statistical Process Control (SPC)

Key points

Where AiVibe comes in

Related terms

Terms that refer to Statistical Process Control (SPC)