Proficiency testing schemes for sampling

Importance of the topic

Proficiency testing (PT) schemes for sampling address a critical and often underappreciated source of measurement error: the process of obtaining representative samples. Robust sampling PT helps laboratories and field teams detect procedural weaknesses, quantify the contribution of sampling to overall measurement uncertainty, and supports training and continual improvement. For regulated testing, environmental monitoring, food safety and industrial quality control, assessing sampling competence is essential to ensure that analytical results are meaningful and comparable.

Objectives and overview

This leaflet provides practical guidance on applying ISO/IEC 17043 to PT schemes where sampling is the primary focus. It defines three types of sampling PT schemes, indicates which parts of the ISO/IEC 17043 standard need particular attention for sampling, and highlights key design and operational issues (personnel competence, site preparation, homogeneity, statistical design, and confidentiality). The overall aim is to enable PT providers to organise sampling exercises that produce interpretable performance evaluations and improve sampling practices.

Methodology and test design

The document categorises sampling PT schemes into three types and summarizes the recommended approach for each:

• Type 1 – Procedure-only evaluation: The scheme assesses participants’ sampling techniques or adherence to a prescribed procedure. Performance can be scored using pre-established criteria, expert audits, or deviation checks from a standard procedure. The emphasis is qualitative or on categorical scoring rather than analytical measurement.
• Type 2 – Sampling evaluated; analysis centralized: Participants collect samples which are then analysed by a single, selected laboratory. The PT provider must ensure validated, low-variability test methods are used so that observed variability can reasonably be attributed to sampling rather than analytical method variability.
• Type 3 – Combined sampling and testing: Participant performance is judged on the final analytical results, encompassing both sampling and testing. Participants may analyse samples themselves or on-site. Inclusion of an appropriate reference material (preferably a certified reference material) for each participant allows estimation of analytical bias and separation of sampling vs analytical contributions.

Key technical and organisational considerations when applying ISO/IEC 17043 to sampling PTs include:

• Personnel competence: Demonstrable knowledge in sampling planning, techniques, and site preparation is essential for both organisers and participants.
• Environment and equipment: Environmental conditions (weather, temperature, flow in dynamic media) and site infrastructure must be considered in design, evaluation or mitigated to reduce their impact.
• Planning and item handling: Concepts of production, QC, storage and distribution of PT items translate into requirements for preparing and managing sampling sites and subsequent sample transport and storage.
• Site preparation and homogeneity: The site must be prepared so all participants face comparable sampling challenges. Dynamic systems (rivers, weather-exposed sites) are inherently non-homogeneous and require special design or acknowledgement of increased variability.
• Minimising cross-participant effects: Prior sampling actions (e.g. drill holes, disturbance) must be controlled to avoid biasing results for later participants.
• Statistical design: ISO 13528 should guide statistical planning. The design must separate sampling procedure variability from analytical method variability where possible and account for transport effects.
• Assigned values and evaluation criteria: Methods to determine assigned values depend on PT type; evaluation criteria should include pre-sampling (container choice) and post-sampling (storage, transport) considerations. Scoring schemes can be qualitative or converted to numerical scales for statistical analysis.
• Confidentiality and collusion: Simultaneous on-site sampling prevents true confidentiality; participants must be informed and reasonable measures taken to discourage collusion.

Main findings and discussion

The leaflet clarifies that ISO/IEC 17043 is applicable to sampling PT schemes when participant comparison and objective performance evaluation exist. Practical implications highlighted are: the need to design schemes that isolate sampling variability (Type 2 designs) when the intent is to assess sampling alone; the value of reference materials to quantify analytical bias in combined assessments (Type 3); and the limitations of dynamic or non-homogeneous sampling sites. It also stresses that many ISO/IEC 17043 clauses retain their intent but require tailored interpretation for sampling contexts, particularly for homogeneity, stability and distribution of PT items understood as the sampling site plus collected samples.

Benefits and practical applications

• Quality improvement: Sampling PTs identify weak points in field procedures and inform corrective training or protocol changes.
• Quantifying uncertainty contributors: When properly designed, schemes allow partitioning of sampling and analytical contributions to overall measurement uncertainty.
• Training and education: Hands-on or workshop-style PTs are highly effective for raising participant competence and harmonising practices.
• Regulatory and accreditation support: Sampling PTs provide evidence for competence in accreditation processes and demonstrate adherence to good practice for clients and regulators.

Future trends and potential applications

• Advanced statistical models: Increased use of hierarchical and mixed-effect models to better separate sampling from analytical variability, and to incorporate covariates such as time, environmental conditions and transport effects.
• Remote and digital support tools: Use of mobile data capture, geotagging, and time-stamped photographic evidence to improve traceability of sampling events and to support qualitative scoring.
• Simulated or synthetic PT sites: Development of controlled, reproducible sampling simulators or standardised matrices to improve homogeneity while retaining realistic sampling challenges.
• Expanded use of reference materials: Wider provision of matrix-matched certified reference materials distributed to participants to improve the estimation of analytical bias in combined schemes.
• Collaborative workshop models: Greater emphasis on interactive PTs combining practical exercises with immediate feedback, peer learning and targeted training to accelerate competence development.

Conclusion

Sampling-focused PT schemes are an essential complement to analytical PT. Proper application of ISO/IEC 17043 (with statistical guidance from ISO 13528) enables PT providers to design assessments that either isolate sampling variability or evaluate the combined sampling-plus-analytical process. Careful attention to site preparation, homogeneity, personnel competence and thoughtful statistical design improves the value of these schemes for quality assurance, uncertainty budgeting and professional development.

References

1. ISO/IEC 17043:2010 Conformity assessment — General requirements for proficiency testing.
2. ISO 13528:2015 Statistical methods for use in proficiency testing by interlaboratory comparison.
3. Proficiency testing of sampling. AMC Technical Brief 78, 2017. DOI: 10.1039/C7AY90092A.
4. Eurachem guide: Selection, Use and Interpretation of Proficiency Testing Schemes by Laboratories, 2nd edition, 2011.
5. Eurachem guide: Measurement uncertainty arising from sampling, 2nd edition, 2019.
6. Eurachem leaflet: Pre- and post-analytical proficiency testing, 1st edition, 2009.
7. Produced by the EEE-PT group on behalf of Eurachem. First English edition, June 2020.