QUALITY & INSPECTION
Cpk vs. Ppk: Process Capability Explained
The practical difference between Cpk and Ppk, and how to use process capability indices to know if a process can meet spec.
A process can be perfectly stable — every point
inside its control limits, no unusual patterns — and still produce a
steady stream of parts outside the customer’s specification. Control
charts and process capability answer two different questions, and
confusing them is one of the more common mistakes in a quality
department.
Stable isn’t the same as capable
Control limits, used in SPC, are calculated from the process’s own
statistical behavior — they describe what the process actually does.
Specification limits come from the customer or the design, describing
what the part is allowed to be. A process can be in statistical
control relative to its own limits while still centered or spread in a
way that regularly violates the customer’s spec.
Cp vs. Cpk: centering is what separates them
| Index | What it measures |
|---|---|
| Cp | Whether the process spread fits within the spec width, ignoring where the process is centered |
| Cpk | The same spread comparison, but accounting for how centered the process actually is within the spec |
A process can have good Cp and bad Cpk
A process with a narrow, tight spread that’s shifted off-center can
have a high Cp — the spread fits comfortably within spec width — but a
poor Cpk, because that tight spread is crowded up against one spec
limit. That gap between Cp and Cpk is itself diagnostic: a large
difference between the two numbers points directly at a centering
problem rather than a variation problem.
Ppk: the long-term counterpart
Cpk is typically calculated from short-term, within-subgroup
variation collected during a controlled study. Ppk uses the overall,
long-term variation across all the data collected, including shift
changes, material lot changes, and other real-world sources of drift.
A process with good Cpk but poor Ppk suggests it performs well in a
tightly controlled study but degrades under normal production
conditions over time.
Where training fits
Process capability analysis fits inside the broader quality systems
work covered in SCMEP’s Quality and
Inspection training catalog. As a NIST
Manufacturing Extension Partnership affiliate serving South Carolina
manufacturers since 1989, our focus is connecting Cpk and Ppk
numbers to what they actually mean for your process, not just the
formula.
If your team needs to calculate process capability for a customer
submission or diagnose a centering problem, you can
browse the Quality and Inspection
training catalog or email the training team.
Process capability indices work best on characteristics an FMEA has already flagged as high-risk. See our guide to FMEA — the AIAG-VDA failure mode and effects analysis method.
Frequently asked questions
What’s the difference between control limits and specification limits?
Control limits are calculated from a process’s own statistical behavior and describe what it actually does. Specification limits come from the customer or design and describe what the part is allowed to be.
What’s the difference between Cp and Cpk?
Cp measures whether the process spread fits within the spec width, ignoring centering. Cpk measures the same spread comparison but accounts for how centered the process actually is within the spec limits.
What does a large gap between Cp and Cpk indicate?
A large difference between Cp and Cpk points to a centering problem rather than a variation problem — the process spread may be fine, but it’s shifted off-center within the spec range.
How does Ppk differ from Cpk?
Cpk is typically calculated from short-term, within-subgroup variation during a controlled study. Ppk uses long-term variation across all production data, including shift changes and material lot changes.
