Cp, Cpk, Pp and Ppk explained
Published
What the four process capability indices measure, how they differ, and what a gap between Cpk and Ppk tells you.
What process capability measures
Process capability compares how much a process varies with how much variation the specification allows. For a characteristic with a lower specification limit (LSL) and an upper specification limit (USL), the allowed spread is USL − LSL.
Cp and Cpk
Cp = (USL − LSL) / (6σ) describes potential: it ignores where the process is centred. Cpk = min((USL − μ) / 3σ, (μ − LSL) / 3σ) accounts for centring and is never larger than Cp. For one-sided specifications only the relevant side is used.
In Cp and Cpk, σ is a short-term, within-subgroup estimate — for individual measurements it is commonly estimated from the average moving range divided by 1.128.
Pp and Ppk
Pp and Ppk use the same formulas but with the overall standard deviation of all data, so they include drift and shifts between subgroups. They describe the performance actually delivered over the period.
Why the gap matters
A Cpk that is high while Ppk is much lower means the process is capable in the short term but its mean is moving. Tool wear is a common cause. This is why drift detection and correlation with tooling matter as much as the index values.
Qynetic computes these indices with deterministic, automatically tested code. They are not produced by a language model.