Key takeaways
Short answer: Cp and Cpk are the two core process-capability indices, and the difference is centering. Cp measures whether a process is precise enough — whether its spread fits within the tolerance — but ignores where the process is centered. Cpk adds centering: it measures whether the process both fits within the tolerance and sits in the right place. Because Cpk accounts for centering and Cp does not, Cpk is always less than or equal to Cp, and a gap between them reveals an off-center process. For the stability prerequisite, see in control vs in spec.
Cp — process capability — measures whether a process is precise enough for its tolerance: is the natural spread of the process narrow enough to fit inside the specification limits? It compares the width of the tolerance to the width of the process variation, ignoring entirely where the process is centered. A Cp of 1.0 means the process spread exactly fills the tolerance; higher than 1.0 means the spread is comfortably narrower than the tolerance, with room to spare. Cp answers a single question: is this process tight enough that it could fit within the tolerance? What it deliberately does not consider is whether the process actually is centered within the tolerance — Cp would give the same value to a process perfectly centered and one shifted hard against one limit, as long as the spread is the same. That blind spot is exactly what Cpk fixes.
Cpk measures process capability while accounting for centering — it asks not just whether the process is narrow enough to fit, but whether it actually fits where it sits. Cpk looks at how close the process is to the nearest specification limit, so a process shifted toward one limit scores lower even if its spread is unchanged. A Cpk of 1.0 means the process, as centered, just reaches a limit; higher means it sits comfortably within both limits; a low Cpk means part of the process is at or beyond a limit, producing defects. Cpk is the more complete and more honest measure, because it captures both sources of trouble: a process can fail either by being too wide (a Cp problem) or by being off-center (a centering problem Cp misses but Cpk catches). Cpk reflects the real defect risk.
The mathematical relationship is fixed: Cpk is always less than or equal to Cp. The two are equal only when the process is perfectly centered in the tolerance — at that point, fitting and being centered amount to the same thing. As the process drifts off-center, Cpk falls below Cp, and the size of the gap between them measures exactly how off-center the process is. This is what makes reading the two together so useful: Cp tells you the best the process could do if it were centered, Cpk tells you what it is actually achieving where it sits, and the difference tells you how much you could gain simply by re-centering. A large gap is a signal that the easiest improvement available is not reducing variation but moving the process back to the middle of the tolerance.
A process has a spread comfortably narrower than the tolerance — its Cp is 1.5, meaning it is precise enough to fit with room to spare. But the process is running shifted toward the upper limit, so its Cpk is only 0.9 — part of the distribution is at or past the upper limit, producing scrap. The high Cp says the process is capable; the low Cpk says it is nevertheless producing defects, because it is off-center. The fix here is not to make the process more precise (the Cp is already good) but simply to re-center it — shift the mean back to the middle of the tolerance, and the Cpk rises toward the Cp of 1.5, eliminating the avoidable scrap. Reading Cp alone, you would think the process was fine; the gap to Cpk revealed the off-center problem and pointed at the cheap fix.
The practical value comes from reading both indices side by side. A high Cp and a high Cpk together mean a process that is both precise and well-centered — genuinely capable. A high Cp with a low Cpk means a precise but off-center process: the variation is fine, but the process needs re-centering, usually a cheap adjustment. A low Cp (and therefore necessarily a low Cpk) means the process is fundamentally too wide for the tolerance, and re-centering will not save it — you need to reduce variation, a harder, more fundamental fix. So the pair diagnoses the problem and points at the remedy: the gap between them shows the centering opportunity, and the absolute level of Cp shows whether variation reduction is also needed. One number cannot do this; the two together can.
Cp and Cpk quantify how reliably a process produces in-tolerance parts, which is the foundation of the quality factor of OEE. A capable, centered process (high Cpk) produces few defects, supporting a high quality factor; a low Cpk means systematic scrap that drags it down, the same yield and scrap reality. Crucially, capability indices are only valid once the process is in statistical control — an unstable process has no stable spread to measure — which ties Cp and Cpk to common versus special cause variation. Capability is the upstream measure of what OEE then tracks continuously: a process with strong Cpk starts production with a healthy quality factor.
Fabrico tracks the live good-versus-defective outcome that capability indices predict — confirming, in everyday production, whether a process certified as capable is actually holding its tolerances. By trending the quality factor and scrap over time, it reveals when a process has drifted off-center (a falling quality factor despite stable equipment) or lost capability, the real-world symptoms of a Cpk problem. It turns a one-time capability study into ongoing assurance. Book a demo to see process capability play out in live OEE.
Cp measures whether a process's spread is narrow enough to fit within the tolerance, ignoring centering. Cpk measures capability while accounting for centering — whether the process both fits and sits within the tolerance. Cp assumes centering; Cpk does not, so Cpk is always less than or equal to Cp.
Because Cpk accounts for how off-center the process is, while Cp ignores centering. The two are equal only when the process is perfectly centered. As the process drifts off-center, Cpk falls below Cp, and the gap measures how off-center it is.
It means the process is precise enough to fit the tolerance (high Cp) but is running off-center (low Cpk), so part of the distribution crosses a specification limit and produces scrap. The fix is usually to re-center the process, not to reduce its variation.
A Cpk of 1.0 means the process, as centered, just reaches a specification limit; higher values mean more margin. Many industries target a Cpk of 1.33 or higher for comfortable capability, though the required level depends on the application and risk.
They quantify how reliably a process produces in-tolerance parts, the foundation of the OEE quality factor. A capable, centered process (high Cpk) supports a high quality factor, while a low Cpk means systematic scrap. The indices are only valid when the process is in statistical control.
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