What Pp measures
Pp compares the specification width with the overall variation of the process. It is similar to Cp, but it uses long-term performance data rather than a short-term estimate of capability.
Process Capability Guide
What Are Pp and Ppk?
Pp and Ppk are process performance indices that look similar to Cp and Cpk, but they are typically used to evaluate overall performance over a broader time period rather than short-term capability.
What Ppk measures
Ppk adds process centering to the picture, much like Cpk. It shows how close the long-term process mean is to the nearest specification limit while accounting for overall variation.
Pp and Ppk vs Cp and Cpk
Cp and Cpk are often used for short-term process capability, while Pp and Ppk are commonly used for long-term process performance. Comparing both sets of indices can reveal whether capability holds up over time.
Why Pp and Ppk matter
If short-term capability looks strong but long-term performance is weak, the process may be drifting over time due to setup changes, material shifts, environmental conditions, or measurement instability.
Process capability calculator
If you want to move from theory to practice, use the Cp / Cpk calculator to upload data, enter specification limits, and review capability metrics with a histogram and plain-language interpretation.
Try the calculator
Use the calculator on the main page to compare sample data, upload your own measurements, and see how Cp and Cpk change with different specification limits.
Open the calculator