Overview - ISO 18391:2016 (Population specification)
ISO 18391:2016 is a Geometrical Product Specification (GPS) standard that defines rules to establish and indicate population specifications for manufactured parts. Rather than applying to individual workpieces, a population specification sets conditions on population characteristics (statistics calculated from one measured global characteristic per workpiece across a population). ISO 18391 provides the notation, rules and interpretation needed to express statistical hypotheses for tolerancing and verification without mandating how tolerance values are calculated.
Key topics and technical requirements
- Population characteristic: A statistic (e.g., arithmetic mean, standard deviation, quantiles) computed from global individual characteristic values taken one per workpiece.
- Population condition: Limits or constraints that apply to a population characteristic (for example, requiring the mean diameter µ ≤ 10.1).
- Complement to individual specification: By default, a population specification complements an individual GPS specification; both must be satisfied independently unless explicitly stated otherwise.
- Local → global transformation: Local characteristics are transformed to one or two global characteristics via rank‑order operators (see ISO 14405‑1); if none are given, separate global minima and maxima apply.
- Univariate vs multivariate: Individual global characteristics are univariate by default. Orientation and location characteristics are typically multivariate unless explicitly indicated otherwise.
- Notation and indication rules: The standard prescribes how to indicate a population specification alongside individual tolerances and how to define target values (defaults where applicable).
- Scope limit: ISO 18391 does not prescribe calculation methods for tolerances or mandate a particular statistical method - it provides the tools to express population requirements consistently.
Practical applications and users
- Manufacturing engineers and product designers who apply statistical tolerancing to control assembly behavior.
- Quality and metrology specialists implementing statistical process control (SPC) or process capability requirements that reference population-level metrics (mean, σ, quantiles).
- Inspection and verification teams that need to document and verify both individual and population-level GPS requirements.
- Standards developers and technical committees integrating GPS requirements within an ISO GPS matrix.
Practical examples in the standard include specifying population limits on means and standard deviations of feature sizes and transforming local measurement data to global characteristics for population analysis.
Related standards
Keywords: ISO 18391, population specification, Geometrical Product Specifications, GPS, population characteristic, statistical tolerancing, SPC, metrology, univariate, multivariate.
