Running a process capability study
A process capability study measures how well a stable process fits within its specification limits, expressed as capability indices such as Cp and Cpk. It tells a plant whether a process is inherently able to make conforming product, and where its output sits relative to the tolerance, before any tightening of control is attempted.
What it is
Capability compares the natural spread of a process to the width of its specification. The study collects data from a process already in statistical control, calculates the spread and centring, and reports indices that quantify the margin — or lack of it — between normal variation and the limits.
Why it is done
Reacting to individual out-of-spec parts without knowing capability leads to constant firefighting. A capability study reveals whether the process is fundamentally capable (needs only better control) or incapable (needs redesign or equipment change), directing improvement effort correctly.
How it is done
The process is first confirmed to be stable using control charts, because capability is meaningless on an unstable process. A representative sample is measured, the spread and mean computed, and capability indices calculated against the specification. The result classifies the process and points to the right action — re-centre, reduce variation, or change the process.
- Confirm stability
- Define spec limits
- Collect sample data
- Compute spread & mean
- Calculate Cp/Cpk
- Decide action
What to watch for
Calculating capability on an out-of-control process gives a meaningless number — stability must come first. Confusing Cp (spread only) with Cpk (spread and centring) hides an off-target process behind a healthy-looking figure.
Related practices
Reducing changeover time with SMED
Optimising clean-in-place (CIP)
Setting up an OEE measurement programme
Related topics
Common in: Pharmaceuticals · Chemicals · Food Processing · Steel & Metals · Paper & Packaging