Proficiency testing schemes for sampling

Significance of the topic

Proficiency testing (PT) schemes for sampling address a frequently overlooked but critical source of measurement error: the sampling stage. Effective sampling PT schemes help laboratories and organizations identify weaknesses in field procedures, quantify the sampling contribution to overall uncertainty, and drive improvements through education and harmonized practice. They are particularly valuable for regulated industries, environmental monitoring, food safety, and any application where representative sampling is essential for reliable analytical results.

Objectives and overview of the guidance

The leaflet provides practical guidance on applying ISO/IEC 17043 principles to PT schemes that involve sampling. Its primary objective is to clarify when ISO/IEC 17043 is applicable to sampling exercises and to outline the specific design, operational and assessment considerations that PT providers must address so that participant performance can be fairly and meaningfully evaluated.

Types of PT schemes for sampling

The document distinguishes three principal PT scheme types, each with different evaluation targets and design implications:

• Type 1 — Assessment of the sampling procedure alone. Participants are evaluated on adherence to a predefined sampling protocol or against expert audit criteria. Performance can be scored qualitatively or with a structured numerical scoring system.
• Type 2 — Sampling by participants with analysis centralized to a single reference laboratory. The provider ensures validated, low-variability test methods so that observed between-participant variability is attributable primarily to sampling rather than analytical variability.
• Type 3 — Combined assessment of sampling and testing. Participant performance is judged on analytical results obtained either on-site or in their laboratory. Providing an appropriate reference material (ideally a certified reference material) enables the separation or combined assessment of sampling bias and analytical bias.

Methodology and key considerations for applying ISO/IEC 17043

When adapting ISO/IEC 17043 to sampling PTs, several clauses require particular attention and operational interpretation. The leaflet highlights practical aspects for PT providers to control and document:

• Personnel: Demonstrate competence in planning and executing sampling exercises, and in preparing sampling sites and instructions.
• Equipment, accommodation and environment: Identify environmental variables (e.g., rain, wind, temperature) that can affect comparability and either include them in the performance evaluation or minimize their influence.
• Planning: Treat production, quality control, storage and distribution responsibilities as requirements for preparing equivalent sampling sites and for post-sampling handling/transport.
• Preparation of PT items: Prepare sampling sites to deliver an equivalent challenge to each participant; control or document transient influences and previous participant impacts (e.g., drilled holes, trampling).
• Homogeneity and stability: Aim for maximum similarity of the sampled item across participants. Dynamic matrices (rivers, plumes) pose inherent homogeneity and stability challenges and may limit the type of meaningful comparisons.
• Statistical design: Use ISO 13528 guidance for interlaboratory PT statistics, and explicitly consider the distinction between variability from sampling versus analytical methods. Account for transport-induced changes where relevant.
• Assigned value and evaluation criteria: Define how assigned values are determined according to PT type (procedural scoring, single-lab reference results, or combined result with reference materials). Include pre-sampling (containers) and post-sampling (storage/transport) aspects in the evaluation rules.
• PT item handling and storage: Recognize that the PT item comprises both the sampling site and the physical samples recovered; document handling and storage conditions comprehensively.
• Packaging, labelling and distribution: Provide clear instructions when specific packaging and labelling are required for sample integrity; where direct measurement at the site occurs, these aspects may be less applicable.
• Data analysis and records: For procedure-based evaluation, qualitative comparisons may suffice; alternatively, convert observed deviations into numerical scores (e.g., ordinal scores for negligible/minor/major deviations) and apply appropriate statistical analyses.
• Confidentiality and collusion: If participants sample simultaneously at the same site, inform them that confidentiality cannot be fully guaranteed and implement measures to reduce collusion risk.

Main findings and discussion

The leaflet synthesizes practical implications: sampling PTs are feasible under ISO/IEC 17043 when the provider establishes a transparent mechanism for comparison and evaluation. The appropriate PT design depends on whether the intention is to assess sampling technique alone, sampling with centralized analysis, or the entire sampling-to-result chain. Key challenges are achieving equivalent challenges for all participants, separating sampling from analytical variability, and accounting for dynamic environmental factors. Use of a single, well-validated laboratory or of certified reference materials helps isolate sampling effects from analytical bias.

Statistical design and judgement criteria must be fit-for-purpose; ISO 13528 provides a well-established statistical framework but must be applied with an explicit understanding of the sampling-specific sources of variability. Participant education (workshops, feedback) substantially enhances the value of PT schemes beyond simple pass/fail outcomes.

Benefits and practical applications

Sampling-focused PT schemes provide multiple practical benefits:

• Identify and quantify sampling-related contributions to measurement uncertainty.
• Benchmark and harmonize field practices across organizations and laboratories.
• Detect systematic procedural flaws and enable corrective actions through training and workshops.
• Support accreditation and quality assurance by demonstrating competence in the sampling stage.
• Improve confidence in downstream analytical results by ensuring representative and properly handled samples.

Future trends and potential applications

Emerging directions that can enhance sampling PTs include:

• Increased use of certified reference materials and synthetic test sites to provide stable, comparable challenges.
• Development of hybrid or virtual PTs (e.g., digital reporting, simulated matrices) to complement field exercises when logistics or safety limit in-person sampling.
• Stronger integration of real-time monitoring, sensors and data logging to document environmental conditions and transport histories.
• Advanced statistical and modelling approaches to deconvolute sampling versus analytical variability and to estimate measurement uncertainty contributions more robustly.
• Expanded training and interactive feedback mechanisms (on-site workshops, video coaching) tied to PT outcomes to accelerate capability development.
• Tighter harmonization of PT design and evaluation criteria across national and international PT providers to improve comparability.

Conclusion

Proficiency testing schemes that address sampling are essential for a complete assessment of measurement quality. Applying ISO/IEC 17043 to sampling PTs is appropriate when a clear comparison and evaluation mechanism exists. Successful schemes require careful planning of site preparation, control of environmental influences, robust statistical design (ISO 13528), and transparent evaluation rules that reflect whether sampling, analysis, or both are under scrutiny. Beyond performance assessment, sampling PTs provide important educational value and enable better characterization of the sampling contribution to overall measurement uncertainty.

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, Selection, Use and Interpretation of Proficiency Testing Schemes by Laboratories, 2nd edition, 2011.
5. Eurachem, Measurement uncertainty arising from sampling, 2nd edition, 2019.
6. Eurachem Leaflet on Pre- and post-analytical proficiency testing, 1st edition, 2009.
7. Produced by EEE-PT group on behalf of Eurachem. First English edition, June 2020.