ISO 13528:2022

Overview

ISO 13528:2022 - Statistical methods for use in proficiency testing by interlaboratory comparison - is the international guideline for the statistical design, analysis and interpretation of proficiency testing (PT) schemes. The third edition (2022) provides detailed, practical methods for proficiency testing providers, laboratories, inspection bodies and accreditation bodies to design PT studies, determine assigned values and uncertainties, evaluate participant performance, and report results. The standard covers both quantitative measurements and qualitative (nominal/ordinal) observations, and includes guidance on robustness, outlier handling and graphical presentation.

Key topics and technical requirements

ISO 13528 includes comprehensive statistical topics and procedural requirements, such as:

• Statistical design of PT schemes: guidance on choosing sample sizes, reporting formats, handling censored data, and designs for small numbers of participants.
• Initial review of items and results: checks for homogeneity, stability and method-dependent considerations.
• Robust statistical methods & outlier detection: procedures for blunder removal, robust estimators and formal outlier techniques.
• Assigned value and uncertainty: methods to determine assigned values (reference, single-lab, expert consensus, participant consensus) and to estimate their standard uncertainty.
• Criteria for performance evaluation: approaches for setting evaluation criteria including experience-based, expert judgement or model-based limits.
• Performance statistics: calculation and interpretation of z‑scores, z′‑scores, zeta (ζ) scores, E‑scores, and methods to assess participant-reported uncertainties.
• Graphical methods: Youden plots, histograms, kernel density plots, plots of repeatability and combined score visualizations.
• Qualitative PT: design and scoring for nominal and ordinal data, and assessment of expert judgments when compared objectively to reference or consensus values.
• Annexes and tools: informative annexes with robust analysis, examples, bootstrapping code and bibliographic references.

Practical applications and users

ISO 13528 is essential for:

• Proficiency testing providers designing and validating PT schemes and reports.
• Accreditation bodies using PT results to verify laboratory competence and conformity to requirements.
• Laboratories and inspection bodies participating in PT to demonstrate fitness-for-purpose, validate uncertainty claims and support continuous improvement.
• Regulators and purchasers who need objective evidence of measurement comparability.

Practical uses include interlaboratory method validation, monitoring laboratory performance over time, assessing measurement bias and precision, and scoring qualitative inspections or classifications.

Related standards

• ISO/IEC 17043 (Conformity assessment - General requirements for proficiency testing) - ISO 13528 provides the statistical backbone to meet ISO/IEC 17043 expectations.
• Other ISO guides on measurement uncertainty and reference materials are complementary when determining assigned values and uncertainties.

Keywords: ISO 13528, proficiency testing, interlaboratory comparison, statistical methods, assigned value, z‑score, ISO/IEC 17043, proficiency testing providers.