How is the supplier capability gap calculated?

Required index minus measured Ppk, signed so positive means short: a 1.33 requirement against Ppk 1.12 is a gap of 0.21. Ppk is the basis because it includes the drift the customer receives; Cpk minus Ppk is the stability gap.

What is the difference between Cpk and Ppk?

Cpk uses within-subgroup variation, the short-term spread the process holds between disturbances. Ppk uses overall variation including drift, setups and material lots. Ppk sits at or below Cpk apart from sampling noise.

Why put a confidence bound on Ppk?

Because an index is an estimate from a sample. Bissell's one-sided 95% bound says Ppk 1.12 on 60 measurements has demonstrated only about 0.94. A study that brushes the requirement while its bound sits below has not demonstrated it.

How many parts should a capability study have?

Enough that the lower bound, not just the point estimate, clears the requirement. That usually means 100 measurements or more; 25-piece studies are screening evidence, and the page warns below 30 because the bound itself becomes strained.

Supplier Quality, Development & Audits calculator

Supplier Capability Gap Calculator: Required Index Against Demonstrated Ppk

Measure a supplier's capability shortfall in the units capability has. Enter Cpk, Ppk, the required index and sample size; the gap, stability split and 95% lower bound come back.

What this calculator does

Measure a supplier's capability shortfall, split drift from spread and bound the estimate.

Formula used

Capability gap = required index − measured Ppk (positive = short)
Stability gap = measured Cpk − measured Ppk
Ppk lower bound (95%, one-sided) = Ppk × (1 − 1.645 × √(1 ÷ (9·n·Ppk²) + 1 ÷ (2(n − 1))))
Sigma level = 3 × Ppk

Inputs explained

Supplier's Measured Cpk: Within-subgroup index from the study, the short-term potential.
Supplier's Measured Ppk: Overall index including drift; the gap is measured against it.
Required Capability Index: The bar the characteristic must meet.
Study Sample Size: Measurements behind the indices, setting the bound width.

How to use the result

Best suited to dispositioning a PPAP capability study, separating drift from spread in a corrective action, checking whether a study demonstrated the bar.
An index summarizes a distribution; the histogram is not optional. The gap prices capability, not conformance: a capable process badly targeted still ships scrap.

Current U.S. benchmarks

U.S. manufacturing runs at 75.7% of capacity (Federal Reserve, Aug 2026). New factory orders are up 8.5% year over year (Census).

Common questions

How is the supplier capability gap calculated? Required index minus measured Ppk, signed so positive means short: a 1.33 requirement against Ppk 1.12 is a gap of 0.21. Ppk is the basis because it includes the drift the customer receives; Cpk minus Ppk is the stability gap.
What is the difference between Cpk and Ppk? Cpk uses within-subgroup variation, the short-term spread the process holds between disturbances. Ppk uses overall variation including drift, setups and material lots. Ppk sits at or below Cpk apart from sampling noise.
Why put a confidence bound on Ppk? Because an index is an estimate from a sample. Bissell's one-sided 95% bound says Ppk 1.12 on 60 measurements has demonstrated only about 0.94. A study that brushes the requirement while its bound sits below has not demonstrated it.
How many parts should a capability study have? Enough that the lower bound, not just the point estimate, clears the requirement. That usually means 100 measurements or more; 25-piece studies are screening evidence, and the page warns below 30 because the bound itself becomes strained.

Last reviewed 2026-10-01.