Overview
SIST ISO 22514-4:2017 - "Statistical methods in process management - Capability and performance - Part 4: Process capability estimates and performance measures" defines commonly used process capability and process performance measures. The standard explains basic concepts (location, dispersion, mean square error, reference limits), presents capability and performance indices for both normal and non‑normal distributions, and describes procedures for estimating and reporting these measures. It also encourages reporting confidence intervals and distinguishing capability (statistically stable processes) from performance (processes not yet stable).
Key topics and technical requirements
- Fundamental concepts: definitions of location, inherent/total/short‑term dispersion, process spread (reference interval) and mean square error (MSE).
- Capability vs performance: formal separation of capability conditions (stable) and performance conditions (unstable), with corresponding index selection.
- Indices for measured data:
- For normal distributions: capability indices such as C and Cpk; performance indices such as P and Ppk.
- For non‑normal distributions: procedures using probability paper, Pearson curves, and distribution‑identification methods.
- Special cases: methods for unilateral tolerances, one‑sided limits, and when no specification limits are provided.
- Other measures: Process Capability Fraction (PCF), proportion out‑of‑specification assessments, and alternative descriptive methods.
- Estimation & reporting:
- Annexes describe estimating standard deviations, using Pearson curves, distribution identification, and confidence intervals.
- The standard recommends computing and reporting confidence intervals for capability/performance point estimates.
- Scope and references: cross‑references to other parts of ISO 22514 (Parts 1–8) and to machine study methods (ISO 22514‑3).
Practical applications
- Use SIST ISO 22514-4:2017 to:
- Evaluate whether a process can meet specifications (capability studies).
- Monitor ongoing production where statistical stability is not yet achieved (performance assessment).
- Select appropriate capability indices for non‑normal or skewed data using Pearson curves or distribution identification.
- Report robust capability/performance results including confidence intervals for supplier evaluation, regulatory compliance, and internal continuous improvement.
- Typical applications include manufacturing quality control, component and assembly studies, supplier assessments, and Six Sigma or SPC programs.
Who should use this standard
- Quality engineers, process engineers, statisticians, metrology specialists, compliance officers, and continuous improvement managers who perform or review capability and performance studies.
Related standards
- ISO 22514 series (Parts 1–8), notably:
- Part 1: General principles and concepts
- Part 2: Time‑dependent process models
- Part 3: Machine performance studies for discrete parts
- Part 7: Capability of measurement processes
Keywords: SIST ISO 22514-4:2017, process capability, process performance, capability indices, Ppk, Cpk, statistical methods, non‑normal distribution, confidence intervals, process capability measures.
