Overview
EN ISO 18391:2016 - Geometrical product specifications (GPS) - Population specification defines rules to establish and indicate population specifications for geometrical characteristics. Instead of specifying limits for each individual workpiece, this standard specifies how to express requirements and statistical hypotheses that apply to a population of workpieces (a collection), complementary to individual GPS tolerances.
This standard provides the syntax and rules to: describe population characteristics (statistics derived from individual characteristic values), state population conditions (limits on those statistics), and indicate population specifications in documentation and drawings. It does not mandate specific tolerancing calculation methods; its purpose is to provide tools to express population‑level requirements.
Key topics and requirements
- Population vs. individual specification: Population specifications complement individual GPS specifications by expressing statistical hypotheses about a batch or population of parts.
- Population characteristics: Defined as statistics computed from one global individual characteristic per workpiece (examples: arithmetic mean, standard deviation, quantiles, min/max).
- Population conditions: Limits or conditions applied to population characteristics (e.g., mean ≤ value).
- Rules for establishing specifications: Clauses explain how to derive global characteristics from local measurements (using rank‑order operators per ISO 14405‑1) and how population requirements imply or interact with individual specifications.
- Indication and notation: The standard defines how to indicate a population specification on drawings and in documentation (including a specific modifier/symbol and rules in Clause 5 and Clause 6).
- Scope limitations: ISO 18391:2016 is not a recipe for tolerance calculation - it standardizes expression and verification hypotheses only.
- Normative references: Relies on and links to ISO 1101, ISO 14405‑1, ISO 17450‑1 and the GPS matrix model (see Annex A).
Applications
- Statistical tolerancing: Define acceptable population-level metrics (mean, variance, quantiles) when using statistical assembly or process assumptions.
- Statistical process control (SPC): Specify control limits or target metrics for batches of manufactured parts.
- Quality contracts and inspection plans: Express contractual population requirements and acceptance criteria for lot release.
- Design for assembly: Capture population hypotheses when worst‑case individual tolerancing is overly conservative and statistical assumptions are justified.
Who should use this standard
- Design engineers specifying tolerances for production batches
- Metrologists and quality engineers developing inspection and SPC plans
- Manufacturing engineers implementing statistical tolerancing strategies
- Technical authors, procurement and OEMs who must document population-level requirements
Related standards
EN ISO 18391:2016 is essential when you need a consistent, standardized way to express and communicate statistical hypotheses and population‑level GPS requirements across design, manufacturing and quality functions.
