Participation in interlaboratory comparisons forms part of the monitoring of the validity of results in accredited laboratories. In this context, small interlaboratory comparisons (small ILC) can become a useful tool when there is no suitable proficiency testing (PT) scheme available, when the number of laboratories in the sector is very small, or when the technical characteristics of the measurement make a conventional PT impractical. The key criterion is not that the exercise is small, but that it is technically appropriate, well organised and capable of reliably assessing the comparability of results.
This type of exercise is not intended as a general substitute for a PT scheme in accordance with ISO/IEC 17043, nor as a simplified route to avoid more demanding requirements. Its purpose is to cover situations in which an interlaboratory comparison among a small number of participants is the reasonable option, provided that it is properly justified within the laboratory’s quality strategy.
What is considered a small ILC
A small interlaboratory comparison is an interlaboratory comparison organised by, and among, seven or fewer laboratories, including the organiser or organisers. Although in practice the most common exercises involve between two and four participants, the document sets that maximum of seven in order to define the scope of this category.
In addition, these guidelines are mainly intended for quantitative small ILCs within the accreditation process. They are not aimed at PT schemes organised by PT providers, nor are they intended to serve as a route for the accreditation of small proficiency testing schemes instead of applying the logic of ISO/IEC 17043 where appropriate.
When it may make sense to participate in a small ILC
Not all laboratories need to resort to a small ILC, but there are situations in which this option may be fully justified. For example:
- when there is no suitable PT scheme available on the market;
- when the measurement belongs to a field with very rapid technical development;
- when the tests or measurements are highly specialised;
- when the number of laboratories carrying out that activity is very low;
- or when participation in a conventional PT would impose a disproportionate burden on the laboratory.
That said, the decision to use a small ILC should not be taken simply for convenience. The approach itself requires that suitable PT schemes on the market must first have been carefully evaluated, and that it must genuinely be concluded that they are not appropriate for that specific need.
How to justify a small ILC in accreditation
One of the most important aspects of a small ILC is that, unlike a PT organised by a competent provider, it is not only the laboratory’s result that is examined. It is also necessary to assess how the exercise was organised, in order to verify that the relevant requirements that provide confidence in the comparison have been taken into account.
This especially affects two situations:
- when the laboratory has organised and participated in the small ILC;
- and when the laboratory has only participated, but must be able to justify why it considered it appropriate for its purpose.
In other words, if a laboratory relies on a small ILC to support the validity of its results, it must be able to explain not only what result it obtained, but also why that exercise deserves confidence.
Performance evaluation: why a small ILC is not interpreted in the same way as a large PT
The statistical evaluation of a small ILC is usually more complex than that of a comparison with many participants. As the number of laboratories decreases, it becomes more difficult to identify the distribution of results, reliably detect outliers, or derive with confidence an assigned value and a standard deviation for proficiency assessment (SDPA) based solely on participants’ results. For this reason, it is generally not recommended to base the assigned value and the SDPA only on reported results, except in very specific cases and with sufficient technical and metrological experience.
From a metrological point of view, the preferred approach is clear:
- use an assigned value based on an external reference;
- secondly, consider with great caution an assigned value based on participants’ results;
- and, if there is not enough basis for that, opt for a comparison without a formal performance score, focused on the technical discussion of results.
Three scenarios for interpreting a small ILC
The technical approach can be summarised in three main scenarios.
1. Small ILC with an external assigned value
This is the most robust scenario. The organiser uses an assigned value based on an external reference, such as a CRM, a measurement standard, results from an expert laboratory, or data from an earlier comparison on the same or a similar material. In this case, performance evaluation can be more direct and may rely, for example, on z scores, En numbers or zeta scores, depending on the nature of the data and the available uncertainties.
2. Small ILC with an assigned value based on participants’ results
This approach is more delicate and, in general, should not be the first option. It may be considered in specific situations, for example when the participants are experienced laboratories that have already sufficiently harmonised their performance in previous rounds, or when one of them operates at a higher metrological level and can provide a sufficiently robust result. Even so, this scenario requires particular caution.
3. Small ILC without an assigned value
When there is no suitable external reference and it is also not reliable to calculate an assigned value from the set of results, the correct approach is not to calculate a formal performance score. That does not mean the exercise is useless. It may still be valuable if it is used to review reproducibility, repeatability, distribution of results, information from extreme values, and the consistency of declared uncertainties. In these cases, the comparison has a strong technical and educational component.
What a well-planned small ILC should include
Planning is one of the most important points in the evaluation of a small ILC. There should be a documented plan describing how the exercise will be carried out and including, at a minimum:
- the main contact person;
- the participants involved;
- the measurand or characteristic to be determined;
- the requirements for production, homogeneity and stability of the item;
- information on the use and preparation of the item;
- the schedule of the exercise;
- the method or methods to be used;
- the way in which the comparability of results will be evaluated;
- the performance evaluation criteria;
- and the reporting format.
Without this kind of planning, the comparison loses technical traceability. And without technical traceability, what remains is not a useful accreditation tool, but a more or less friendly exchange of results among colleagues.
Homogeneity, stability and statistical design
Where relevant for the evaluation, there must be documented evidence of the homogeneity and stability of the items used in the small ILC. The adequacy of the statistical design, the way in which the assigned value is established and, where applicable, the SDPA used must also be reviewed.
This point is crucial because a small comparison cannot afford too many methodological weaknesses. If the number of participants is already limited, any problem with the material or with the design affects the usefulness of the exercise even more.
Organisation, personnel and separation of functions
The organisation of a small ILC must be integrated into the laboratory’s management system. Documentation must be controlled according to its procedures, and the activity must appear in internal audits and management reviews.
In addition, the personnel involved must be competent for the assigned tasks. And if the organiser also participates in the small ILC, it is advisable, wherever possible, that the people performing the measurements are not the same as those organising the exercise, in order to avoid bias or prior knowledge of the levels to be determined.
What the final report should contain
The organiser of the small ILC should issue a report including, at a minimum:
- the date of the exercise;
- the contact person;
- the laboratories involved in organising the exercise;
- the identification of the scheme;
- a description of the item;
- the participants’ results;
- the method used to evaluate the comparability of results;
- the comparability of results and/or performance;
- and comments or recommendations arising from the exercise.
This report is not a decorative formality. It is the piece that allows participants, auditors and accreditation bodies to understand what was done, on what basis it was interpreted and what real usefulness the exercise had.
2026 agenda: current programmes
As part of SHAPYPRO’s planning for 2026, several proficiency testing exercises are already scheduled. To consult the full calendar, the 2026 agenda can be downloaded at the following link: https://shapypro.com/2026-agenda/
The programmes currently in progress are:
- EN1656 – P. aeruginosa
- EN1657 – C. albicans
- EN1276 – S. aureus
- EN1650 – A. brasiliensis
- EN13704 – B. cereus
Integrating this type of calendar into the interlaboratory comparison strategy is important because participation does not depend only on technical availability, but also on planning. Choosing the right exercise, the right timing and the right form of participation remains part of serious work, even if there is always someone hoping to sort it out two weeks before the audit.
How SHAPYPRO can help
In this context, SHAPYPRO can help laboratories assess whether a small ILC is truly appropriate within their strategy for ensuring the validity of results, reviewing aspects such as exercise planning, the definition of the assigned value, the evaluation of comparability, the technical documentation and the consistency with the accredited scope. This makes it possible to turn a small comparison into a useful and defensible tool, rather than an improvised solution with methodological appearances.
Conclusion
Small interlaboratory comparisons can be a valid tool within the accreditation process when there is no suitable PT available or when the nature of the activity makes it reasonable to rely on a comparison among a small number of laboratories. But their usefulness depends on them being well justified, well designed and well documented. In a small comparison, precisely because there are fewer participants, there is less room for improvisation and a greater need for rigour.
In other words, a small ILC can be perfectly appropriate. What is not usually appropriate is to organise it without method, interpret the results too cheerfully and expect accreditation to see it as a sign of technical creativity.