Let’s call a PT scheme a PT scheme!

Significance of the topic

Interlaboratory comparisons (ILCs) and proficiency testing (PT) schemes are central to assuring measurement quality, demonstrating competency, and supporting accreditation across testing and calibration laboratories. Clear, harmonised terminology is essential to avoid misunderstandings among laboratory staff, accreditation bodies and stakeholders, to ensure correct design and interpretation of comparisons, and to support traceability and comparability of results across sectors.

Objectives and overview of the leaflet

This leaflet aims to clarify basic terminology for ILCs, highlight commonly misused or ambiguous colloquial terms, and promote harmonisation by pointing readers to relevant international standards and guidance. It summarizes the formal definition of an ILC, outlines the principal objectives encountered in practice, distinguishes special types of comparisons, and explains why consistent usage matters.

Methodology and scope

The document presents a conceptual and terminological review rather than empirical research. It draws on established international standards and guidance (notably ISO/IEC 17043 and ISO 13528), IUPAC recommendations and recognised sectoral guides, to:

• Define an ILC as the organisation, execution and evaluation of measurements or tests on the same or similar items by two or more laboratories under predetermined conditions.
• Identify common objectives for ILCs, such as evaluating laboratory performance (PT), assessing a candidate standard method (method performance or collaborative study), characterising properties for reference material certification (material certification study), or demonstrating metrological capability (key and supplementary comparisons).
• Discuss ambiguous colloquial terms (ring test, round robin, circle analysis) and illustrate how they may imply specific designs or distribution methods that are not universally applicable.

Main results and discussion

The leaflet’s principal conclusions and practical points are:

• Terminology matters. ‘‘Proficiency testing’’ and ‘‘external quality assessment’’ are appropriate when the evaluation of participant performance is the principal objective. Other designations should be used only when they accurately reflect the ILC purpose (for example, ‘‘method performance study’’ for method validation, ‘‘material certification study’’ for reference material work, and ‘‘key comparisons’’ for national metrology demonstrations).
• Colloquial terms can mislead. Terms such as ‘‘ring test’’ or ‘‘round robin’’ have been used in multiple contexts with different designs and thus may confuse participants about logistics, sample distribution or objectives.
• ISO/IEC 17043 clarifies distribution schemes by defining ‘‘sequential’’ (a unique item transferred between participants) and ‘‘simultaneous’’ (similar items from a batch distributed at once) PT schemes; using these definitions reduces ambiguity regarding sample handling.
• Harmonisation is slow but necessary. Inconsistent use of terms across standards, national guidance and languages increases the risk of misinterpretation; explicit cross-referencing to standards and sectoral guides mitigates this risk.

Benefits and practical applications

Clear, standardised terminology and purposeful selection of the ILC label deliver multiple practical benefits:

• Improved communication among laboratories, PT providers, accreditation bodies and end-users, reducing misinterpretation of scheme objectives and logistics.
• Enhanced compliance with accreditation requirements and international standards by aligning scheme design and documentation with recognised definitions.
• Better design and selection of PTs by laboratories and procurement teams, ensuring that an ILC’s stated objective (e.g., method validation vs. proficiency assessment) drives sample preparation, statistical treatment and reporting.
• More consistent and defensible interpretation of results, supporting corrective actions, method development and regulatory decision-making.

Future trends and possibilities for use

Several developments can further reduce terminological confusion and improve ILC utility:

• Broader adoption of harmonised metadata schemas for PT schemes (explicitly declaring objective, scheme type, distribution method, statistical approach) to support machine-readable interoperability and searchability.
• Improved translations and localized guidance that faithfully reflect international standard terminology, reducing inconsistent local usage and training gaps.
• Integration of digital platforms for PT administration, automated statistical evaluation (implementing ISO 13528 methods), and interactive participant feedback to speed corrective actions and knowledge transfer.
• Expanded training and outreach—targeted at laboratory staff and procurement officers—to promote consistent terminology and the rationale for selecting specific ILC designs.
• Continued refinement of standards and sectoral guides to address emerging measurement domains and novel sampling/distribution models.

Conclusion

The leaflet advocates straightforward, precise usage of ILC terminology: designate schemes according to their primary objective and use established standard definitions (for example, call a proficiency testing scheme a PT scheme). Consistent terminology, supported by references to ISO standards and recognised guides, reduces ambiguity, improves scheme design and interpretation, and fosters confidence in interlaboratory results.

References

1. ISO/IEC 17043:2010. Conformity assessment — General requirements for proficiency testing. ISO, Geneva (2010).
2. Horwitz W. Nomenclature of interlaboratory studies (IUPAC Recommendations 1994). Pure & Applied Chemistry, 66(9), 1903–1911 (1994).
3. ISO 5725-2:2019. Accuracy (trueness and precision) of measurement methods and results — Part 2: Basic method for the determination of repeatability and reproducibility of a standard measurement method. ISO, Geneva (2019).
4. ISO Guide 35:2017. Reference materials — Guidance for characterization and assessment of homogeneity and stability. ISO, Geneva (2017).
5. BIPM. Key comparisons, supplementary comparisons and pilot studies. (Information available from the Bureau International des Poids et Mesures.)
6. ISO 13528:2015. Statistical methods for use in proficiency testing by interlaboratory comparison. ISO, Geneva (2015).
7. ISO/IEC 17025:2017. General requirements for the competence of testing and calibration laboratories. ISO, Geneva (2017).
8. Brookman B. and Mann I. (eds.) Eurachem Guide: Selection, Use and Interpretation of Proficiency Testing (PT) Schemes. 3rd ed. Eurachem (2021).
9. ISO Online Browsing Platform. ISO standards and related documents. (Access via the ISO online platform.)