Quality
What is Statistical process control?
Statistical process control uses control charts to tell normal variation from real process changes, so problems are fixed before defects are made. Cp, Cpk explained.
Quick answer
Statistical process control (SPC) is a method that uses statistics, chiefly control charts, to monitor a process and tell normal variation from a real change. When a measurement falls outside the control limits or forms a pattern, the process is investigated before it produces defects. Capability indices such as Cp and Cpk show whether a stable process meets its specification.
Updated 30 October 2026. Written by the Ajinkya Technologies engineering team, reviewed by Amey Kadle.
Statistical process control explained
SPC was developed by Walter Shewhart at Bell Laboratories in the 1920s and spread widely through manufacturing after the Second World War. Its central idea is that every process varies, and that variation has two kinds of cause. Common causes are the many small influences always present; special causes are specific events such as a worn tool, a new material lot or a changed setting.
A control chart plots a measurement over time against a centre line and control limits, typically three standard deviations either side. Points outside the limits, runs on one side of the centre line and trends signal a special cause worth investigating. Reacting to every point inside the limits, by contrast, adds variation rather than removing it.
Capability is a separate question: whether a stable process fits inside the customer specification. Cp compares the specification width with the process spread; Cpk also accounts for how well the process is centred. Modern SPC takes measurements automatically from gauges, CMMs and machine sensors, calculates limits and capability live, and alerts the operator at the moment a rule is broken.
Key concepts
- Common cause versus special cause variation
- Control charts with limits at about three sigma
- Run and trend rules signal a change
- Cp and Cpk measure capability against specification
- Automatic data collection makes SPC real time
Questions
What is the difference between control limits and specification limits?
Control limits come from the process data and show its natural variation. Specification limits come from the customer or design and show what is acceptable. A process can be in control and still not capable.
What is Cpk?
Cpk is a capability index that compares the distance from the process mean to the nearest specification limit with three standard deviations. Higher is better; it accounts for both spread and centring.
Do we need SPC if we inspect every part?
Inspection sorts good from bad after the fact. SPC detects a process drifting before it makes bad parts, so it reduces scrap and rework rather than just finding them.
References
How we help with SPC
We connect gauges, inspection stations and machine sensors to SPC charts on the shop floor, with alerts when a rule is broken and every measurement traceable to the part and batch.
Set the reject threshold yourself and watch escapes trade against false rejects (simulated data).
Related terms
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