Overview
ISO 22514-2:2017 - "Statistical methods in process management - Capability and performance - Part 2: Process capability and performance of time-dependent process models" provides a structured procedure for estimating process capability and performance when product or process characteristics change over time. The standard applies to continuous quality characteristics and categorizes process outcomes into eight time-dependent distribution models, supplying calculation formulae for the relevant statistical measures for each model. It is applicable across industrial and economic sectors and is intended for both large serial datasets and smaller sample series.
Key topics and technical requirements
- Time-dependent distribution models: Classification into models A, B, C and D (with practical subclasses such as A1, A2, C1–C4) based on whether the process mean (location) and variance (dispersion) are constant, change systematically, or vary randomly.
- Eight distribution types: The standard defines eight possible resulting-distribution types and links each with appropriate calculation methods.
- Capability and performance indices: Defines and clarifies indices (symbols such as C, P, pk, Cpk/Ppk variants) and prescribes how to compute them depending on the time-dependent model.
- Calculation methods: Provides formulae for location and dispersion measures for each distribution type and guidance on one-sided specification limits and different calculation approaches.
- Reporting: Guidance on reporting process capability/performance indices and recommendation to compute and present confidence intervals for point estimates.
- Process analysis and model verification: Emphasizes prior process analysis (control charts, ANOVA, probability plots) and the need for statistical software to verify model fit and estimate parameters.
Practical applications
- Assessing process capability and short- and long-term performance for processes where mean or variance vary over time (e.g., trends, lot-to-lot shifts, seasonal effects).
- Selecting appropriate capability indices and calculation methods when instantaneous distributions differ from aggregated (resulting) distributions.
- Producing defensible capability reports for regulatory, supplier quality, continuous improvement, and supplier-customer communication.
- Useful in manufacturing (automotive, aerospace, electronics), pharmaceuticals, chemicals, and service processes where time-dependent behavior affects quality.
Who should use this standard
- Quality engineers, process statisticians, SPC specialists, process improvement teams, and auditors who need to evaluate capability/performance for non-stationary processes.
- Tooling and production engineers implementing control plans where time-varying process behavior must be quantified.
Related standards
Keywords: ISO 22514-2:2017, process capability, process performance, time-dependent process models, capability indices, statistical methods in process management, continuous quality characteristics.
