Quality Statistics & Capability
Quality Statistics Knowledge Center
The Qhubio Quality Statistics Knowledge Center — definitions, interpretation, industry targets, conversions, and the free calculator. Start with the Process Capability Calculator if you already have measurement data.
Last updated · Reviewed by the Qhubio engineering team
What process capability measures
Process capability quantifies how well a stable, in-control process meets its engineering specification. The four indices — Cp, Cpk, Pp, Ppk — translate raw measurement data into a single number that can be compared across machines, plants, and suppliers.
How to use this hub
- Run the numbers: the free Process Capability Calculator computes Cp, Cpk, Pp, Ppk, sigma level, and predicted PPM from your data — no sign-up.
- Interpret a value: open the interpretation pages (Cp vs Cpk, Cpk = 1.33, negative Cpk, …) to understand what your result actually means.
- Match a customer target: the industry pages map Cpk thresholds to automotive, aerospace, medical device, CNC, molding, stamping, and welding requirements.
- Convert between metrics: the conversion pages move between PPM, sigma level, and Cpk.
Typical capability targets
- Cpk ≥ 1.33 — minimum for ongoing production (AIAG SPC).
- Cpk ≥ 1.67 — typical PPAP launch target.
- Cpk ≥ 2.00 — safety- and regulation-critical characteristics.
Frequently asked questions
Quality Statistics Knowledge Directory
Interpret Cp, Cpk, Pp, and Ppk results, map them to industry requirements, and convert between PPM, sigma level, and capability.
Capability Interpretation
Cp vs Cpk: Difference, Formulas, and Interpretation
Cp measures potential capability, Cpk measures actual capability including centering. Learn the difference, formulas, and how to interpret both for production processes.
Process Capability Index Explained
The process capability index family (Cp, Cpk, Pp, Ppk) measures how well a process meets specification. Definitions, formulas, and target values explained.
Process Capability Analysis: Step-by-Step
Process capability analysis evaluates whether a stable process meets specification. Learn the data, normality check, sigma estimation, and reporting steps.
Process Capability Study
A process capability study collects 25–100+ measurements from a stable process to compute Cp, Cpk, Pp, and Ppk for PPAP and SPC reporting.
Negative Cpk: Causes and Corrective Actions
A negative Cpk means the process mean lies beyond the nearest specification limit. Learn what it implies, verify the study, and choose the appropriate containment response.
Cpk Below 1: Risk, PPM, and Recovery
A Cpk under 1.00 means the process produces measurable defects. Learn the PPM rate, sigma level, and the actions required to bring Cpk above target.
Cpk = 1.00: The Bare-Minimum Capability
Interpret Cpk 1.00, including its 3σ nearest-limit distance, statistical assumptions, expected PPM, and why it provides little operating margin.
Cpk = 1.33: The 4σ Target
Interpret Cpk 1.33, its approximate 4σ nearest-limit distance, expected defect rate assumptions, and common—but not universal—production targets.
Cpk = 1.67: The PPAP Launch Target
Interpret Cpk 1.67, its approximate 5σ nearest-limit distance, expected PPM assumptions, and its use in some customer launch requirements.
Cpk = 2.00: Six Sigma Capability
Interpret Cpk 2.00, its 6σ nearest-limit distance, modeled PPM with and without a 1.5σ convention, and when demanding targets may be specified.
Cp = 1.00: Spread Equals Tolerance
Cp = 1.00 means the process spread (6σ) exactly equals the tolerance width. There is no safety margin even when perfectly centered.
Cp = 1.33: Minimum Potential Target
Cp 1.33 means the tolerance is approximately 1.33 times the natural 6σ spread. Interpret it with centering, stability, and the applicable requirement.
Cp = 1.67: PPAP-Level Potential
Cp 1.67 describes strong potential capability, but PPAP acceptance and the required index depend on the customer-specific requirement and study method.
Cp = 2.00: Six Sigma Potential
Cp = 2.00 corresponds to Six Sigma potential capability: the process spread is half the specification width.
Industry Requirements
Automotive Cpk Requirements
Understand common automotive capability reference values and how customer-specific requirements, PPAP, the drawing, and the Control Plan determine the actual target.
CNC Machining Cpk Requirements
Plan capability studies for CNC-machined features while accounting for customer targets, tool wear, fixtures, thermal drift, sampling, and study duration.
Injection Molding Cpk Requirements
Typical injection molding Cpk targets and the multi-cavity, shrinkage, and material-lot variation that drive them.
Medical Device Cpk Requirements
Understand how statistically justified capability criteria can support medical-device process validation without inventing universal ISO or FDA Cpk thresholds.
Aerospace Cpk Requirements
Understand aerospace capability planning for key characteristics while separating AS9100 requirements from customer- and program-specific numerical targets.
Stamping Cpk Requirements
Stamping Cpk targets and the tonnage, die-wear, and coil-lot variation that drive long-run capability. Typical: Cpk ≥ 1.33 ongoing, ≥ 1.67 at PPAP.
Welding Cpk Requirements
Welding Cpk targets for joint strength, penetration, and geometry. Typical: Cpk ≥ 1.33 ongoing, ≥ 1.67 at PPAP; safety joints often require ≥ 2.00.
Process State
Process Mean Outside Specification
When the process mean lies outside a specification limit, Cpk is negative. Verify the study, estimate risk under stated assumptions, and follow the approved reaction plan.
Process Variation Too High
When Cp is low the process spread does not fit the tolerance. Diagnose the common-cause variation sources before chasing centering.
Process Not Centered
When Cpk is far below Cp, the mean has drifted off target. Causes include tool wear, offsets, fixturing, and setup. Step-by-step recovery.
Process Capable and Centered
A capable, centered process (Cpk ≥ 1.33 with Cp ≈ Cpk) is the target state. Move from corrective action to predictive monitoring.
Process Capability Improvement
Practical playbook to improve Cp and Cpk: separate centering from variation, attack dominant variation source, validate with a fresh study.
High Defect Rate Process
If your process is producing thousands of PPM, Cpk is well below 1. Use the calculator to quantify the gap and prioritize containment.
Process Producing Defects: A Capability Diagnosis
When defects are being produced, capability analysis tells you whether centering, variation, or both are to blame — and how to fix it.
Capability Conversion
PPM to Cpk Conversion
Convert nearest-tail PPM to an equivalent Cpk under a stable normal model. Understand why total two-sided PPM cannot be converted without additional assumptions.
Cpk to PPM Conversion
Convert Cpk to modeled nearest-tail PPM under a stable normal distribution, with clearly labeled assumptions for centered two-sided estimates.
Sigma Level to Cpk Conversion
Convert sigma level (Z-score) to Cpk: Cpk = sigma-level / 3 (short term) or (sigma-level − 1.5) / 3 (with the Six Sigma shift).
Cpk to Sigma Level Conversion
Convert Cpk to sigma level (Z-score). Cpk × 3 gives the short-term sigma level; add 1.5 for the long-term Six Sigma convention.
Cp vs Pp: Short-Term vs Long-Term
Cp uses within-subgroup (short-term) sigma. Pp uses overall (long-term) sigma. The difference quantifies process drift.
Cpk vs Ppk: Differences and Reporting
Cpk uses short-term within-subgroup sigma; Ppk uses long-term overall sigma. Customers usually want Ppk for ongoing production.
Process Capability Formulas
All four process capability formulas in one place, with worked examples and the difference between within-subgroup and overall sigma.
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