ISO 21748:2017

Overview - ISO 21748:2017 (Guidance)

ISO 21748:2017 provides practical guidance for evaluating measurement uncertainty using performance data from collaborative studies (ISO 5725‑2). It describes a “top‑down” approach that combines estimates of repeatability, reproducibility and trueness with the principles of the GUM (Guide to the Expression of Uncertainty in Measurement) to produce uncertainty evaluations that are applicable across measurement and testing fields.

Key topics and technical requirements

• Use of collaborative study data: How to apply repeatability and reproducibility estimates from ISO 5725‑2 studies in uncertainty evaluation.
• Trueness (bias) assessment: Guidance on accounting for method trueness and uncertainty associated with bias.
• Combining variances: Procedures for combining intra‑ and inter‑laboratory variance components in line with GUM principles.
• Verification of applicability: Requirements for laboratories to confirm that published method performance data apply to their implementation (control of laboratory bias, verification of repeatability, continued verification).
• Scope and limitations:
  • Applicable to all measurement and test fields where uncertainty must be determined.
  • Does not address use of repeatability data alone (when reproducibility data are absent).
  • Models for intermediate precision (ISO 5725‑3) are not incorporated, though the general approach may be extended.
  • Assumes recognized systematic effects are corrected or removed.
• Practical elements covered: sampling and inhomogeneity, sample preparation, effect of response level on uncertainty, uncertainty budgets, expression of combined standard and expanded uncertainties, and comparison procedures between collaborative‑study performance and GUM‑based uncertainty (Clause 14).
• Supporting material: Informative annexes with approaches, experimental evaluation, and calculation examples.

Practical applications - who uses ISO 21748:2017

• Accredited testing and calibration laboratories (e.g., ISO/IEC 17025) using standard methods validated by collaborative studies.
• Method developers and standards committees who need to document method performance and uncertainty claims.
• Accreditation bodies and assessors evaluating a laboratory’s uncertainty estimation and method implementation.
• Quality managers and metrologists applying a cost‑effective, top‑down uncertainty approach when a measurement model is not available.

Related standards (for context)

• ISO 5725‑2 - Method for accuracy (repeatability and reproducibility) from collaborative studies.
• ISO 5725‑3 - Models for intermediate precision (not incorporated but related).
• GUM (ISO/IEC Guide 98‑3) - Principles for uncertainty evaluation used in combination with ISO 21748 guidance.

Keywords: ISO 21748:2017, measurement uncertainty, repeatability, reproducibility, trueness, ISO 5725‑2, top‑down approach, GUM, laboratory verification.