Contents
What is process stability?
Process stability — also called statistical process stability, in-statistical-control, or in Six Sigma terminology simply a controlled process — is the property of a manufacturing process whose output varies only within its natural, inherent variation, with no assignable causes (special causes) acting on it. A stable process is predictable. An unstable process is not — regardless of how tight its specification limits happen to be. In the ISO 22514 / AIAG SPC framework, stability is the prerequisite for every downstream capability claim (Cp, Cpk, Pp, Ppk); without stability, those numbers are arithmetic, not statistics.
A common mistake, even in plants with Cpk dashboards on the wall, is this: they calculate capability on data from a process that was never stable to begin with. A Cpk of 1.67 from an unstable process is not "good quality." It is a number that describes something that will not exist tomorrow. Process stability is the precondition; capability is the reward.
Stability vs. capability — the distinction that most plants blur
This is the confusion that derails more SPC programmes than any other. Stability and capability are two different questions, measured with two different methods, and a process can fail one while passing the other. Knowing which you have is the difference between a real quality system and a decorative one.
The rule: stability first, capability second — never the other way round. A process must be demonstrated stable before any Cpk calculation is meaningful. Plants that publish capability indices without proving stability first are reporting fiction with a decimal point. Stability is shown on a control chart.
Common cause vs. special cause — the Shewhart distinction that still matters
The operational implication is simple and under-practised: most "corrective actions" on the shop floor are tampering. An operator sees one value trending slightly up, nudges a setpoint, and the next sample reads slightly low — so the setpoint gets nudged back. What started as a stable process with 2-sigma natural variation becomes a process with 4-sigma induced variation, driven entirely by well-meaning intervention. A real SPC programme trains operators not to react to anything inside the control limits.
What breaks process stability — the five recurring special causes
Why manual SPC fails — and what closed-loop changes
The digital stack for real process stability
FAQ
What is process stability?
Process stability is the property of a manufacturing process whose output varies only within its natural, inherent variation, with no assignable (special) causes acting on it. A stable process is statistically predictable: the next output will fall inside the calculated control limits with known probability. It is the foundational concept of Shewhart / SPC and the prerequisite for every capability claim (Cp, Cpk, Pp, Ppk) in ISO 22514 and the AIAG SPC manual.
What is the difference between process stability and process capability?
Stability answers "is the process predictable?" — measured with control charts and run rules. Capability answers "does the predictable process fit inside the customer's specification?" — measured with Cp, Cpk (short-term), Pp, Ppk (long-term). The rule is absolute: stability first, capability second. Calculating Cpk on an unstable process produces a number that describes something that will not exist tomorrow. A process can be stable but not capable (predictable but outside spec), capable but not stable (fits today, will drift tomorrow), both, or neither. Only "both" is worth reporting.
What is the difference between common cause and special cause variation?
Common-cause variation is natural, inherent variation present in every output — the process is doing what it was designed to do. You address it by changing the process itself (tooling, fixtures, materials), never by reacting to individual data points. Special-cause variation is an external, assignable disturbance — tool wear, material batch change, operator change, temperature drift. You address it by investigating and eliminating the specific cause. Reacting to common-cause variation as if it were special cause is called tampering, and it is the single most common way stable processes are made unstable.
How do you prove a process is stable?
Control charts plus run rules. X-bar/R for variables data in subgroups, I-MR for individual measurements, p-chart or u-chart for attribute data. Apply Western Electric or Nelson rules: no points beyond 3σ, no 2-of-3 beyond 2σ, no 6 consecutive increasing or decreasing, no 14 alternating, no 8 on one side of the centreline. If no rules are violated across a sufficient sample (typically 25+ subgroups for short-term studies), the process can be declared stable. If rules fire, the special causes are identified and eliminated before any capability study is run.
What breaks process stability in practice?
Five recurring special causes, each with a distinctive control-chart signature. Tool wear — slow drift of the mean in one direction. Material batch change — step-change in mean at a specific timestamp. Operator or shift change — pattern repeating with shift rhythm. Temperature or environmental drift — daily cyclical pattern. Tampering — increased variation with no corresponding physical change, because someone is adjusting the process on common-cause noise. The fifth is by far the most common in plants that "do SPC" without training operators on when not to react.
Why does manual SPC fail at scale?
Three reasons that can collapse Cpk claims in practice. Sampling rhythm misses actual process variation — hourly sampling on a schedule doesn't catch shift-change effects. Selective re-measurement — operators re-measure points near control limits and record the second reading, quietly biasing the dataset. Tampering — adjustments logged against common-cause variation add induced drift. Automated inline capture tied to an MES eliminates all three: no human in the measurement loop, no selective recording, rules evaluated automatically. Manual SPC at scale is usually not wrong in arithmetic — it is measuring a version of the process that doesn't exist.
How does an MES support process stability?
Automated data capture via OPC UA, MQTT or digital I/O gateway — no operator in the measurement loop.
Related: MES · MES Software · OEE · OEE Software · Production Planning Software · Digital Production Control · Shop Floor Terminal · Digital Manufacturing · Manufacturing Analytics · SYMESTIC Process Data · Alarms · Production Metrics
See process stability in a running MES
In a live demo we show how process values are recorded and evaluated with mean, intervention limits and Cp/Cpk.
- Live OEE
- Downtime and reasons
- Connecting machines and ERP
