Calculate Pp and Ppk process performance indices using overall standard deviation. Assess long-term manufacturing process capability.
Pp and Ppk are process performance indices that use the overall (long-term) standard deviation rather than the within-subgroup (short-term) standard deviation used by Cp and Cpk. They capture the full range of variation experienced by a process over time, including shifts and drifts that occur between subgroups.
Pp measures overall spread relative to tolerance, while Ppk accounts for both spread and centering — paralleling the Cp/Cpk relationship. In practice, Ppk is almost always lower than Cpk because long-term variation includes additional sources (material batches, tool wear, environmental changes) not visible within a single subgroup.
This calculator computes both Pp and Ppk from specification limits, process mean, and overall standard deviation. It also shows the gap between Pp and Ppk to indicate centering, and you can compare these with Cp/Cpk for a complete capability picture.
Precise measurement of this value supports data-driven planning and helps manufacturing professionals make informed decisions about resource allocation and process optimization strategies.
Pp and Ppk reflect actual long-term process performance, making them more realistic than Cp/Cpk for predicting future defect rates. They are required in PPAP submissions and are the first metrics customers examine when evaluating supplier quality. Regular monitoring of this value helps teams detect deviations quickly and maintain the operational discipline needed for sustained manufacturing excellence and competitiveness.
Pp = (USL − LSL) / (6s) Ppu = (USL − X̄) / (3s) Ppl = (X̄ − LSL) / (3s) Ppk = min(Ppu, Ppl) where s = overall (total) standard deviation
Result: Pp = 1.11, Ppk = 1.00
Pp = (10.5 − 9.5) / (6 × 0.15) = 1.0 / 0.9 = 1.11. Ppu = (10.5 − 10.05) / (3 × 0.15) = 0.45 / 0.45 = 1.00. Ppl = (10.05 − 9.5) / (3 × 0.15) = 0.55 / 0.45 = 1.22. Ppk = min(1.00, 1.22) = 1.00.
During Production Part Approval Process (PPAP), suppliers must demonstrate process performance by reporting Pp and Ppk from an initial production run. These indices prove the process can consistently produce parts within specification before mass production begins.
A large gap between Cpk and Ppk indicates significant between-subgroup variation. This could be caused by inconsistent setups, varying raw material properties, or environmental factors. Closing this gap through standardization and control can dramatically improve real-world quality.
Always ensure your data collection period covers the full range of expected conditions: multiple shifts, operators, material lots, and environmental conditions. A Ppk based on a single lot of material under ideal conditions will overestimate actual performance.
Cpk uses within-subgroup σ (short-term), Ppk uses overall s (long-term). Ppk captures additional sources of variation like lot-to-lot differences and machine drift. Ppk ≤ Cpk in most cases.
Ppk is more important for predicting real-world defect rates because it includes all variation sources. Cpk is useful for diagnosing whether the inherent (short-term) process is capable before addressing long-term shifts.
AIAG PPAP requirements typically specify Ppk ≥ 1.67 for initial process studies (minimum 300 pieces from a significant production run). Ongoing capability uses Cpk ≥ 1.33.
Theoretically no, because overall σ ≥ within-subgroup σ. In rare cases with very small sample sizes, sampling error can make Ppk appear slightly higher, but this is a data artifact.
AIAG recommends a minimum of 125 individual readings from 25+ subgroups collected over a representative production period that includes normal variation sources (shifts, material lots, operators). Sharing these results with team members or stakeholders promotes alignment and supports more informed decision-making across the organization.
This means the overall spread fits the tolerance (precision is adequate) but the process is off-center. Adjust the process mean toward the midpoint of USL and LSL to improve Ppk.