This leaflet gives some hints on the application of ISO/IEC 17043 [1] for PT providers organising PT schemes for sampling. If there is a comparison between participants and a mechanism for performance evaluation which meets the objective of the PT scheme for sampling, then ISO/IEC 17043 is applicable.&nbsp;<h4>Types of PT schemes for sampling&nbsp;</h4><h5>Type 1</h5>Only the sampling procedure is taken into consideration and evaluated. Performance assessment can be done through a preestablished scoring system or set of criteria. The performance can be assessed by deviations from a standard procedure or through an audit process where experts judge the performance of the participant.&nbsp;<h5>Type 2</h5>Samples collected by the participants are tested by a single laboratory chosen by the PT provider who must ensure that validated test methods with low variability are used. Thus, the variability obtained is attributed to the sampling and not to the test method.&nbsp;<h5>Type 3</h5>The performance of the participant is based on the testing results, and comprises both sampling procedures and test methods. Here the participant can perform the test at the sampling site or at their laboratory. The use of an additional appropriate reference material, ideally a certified reference material, provided by the PT provider to each participant, enables the analytical bias to be determined. Thus, the performance assessment is based on the sampling procedures and test methods combined or separately.&nbsp;<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Eurachem_Proficiency_testing_schemes_for_sampling_3_73f9231ef6.jpg" alt="Eurachem: Proficiency testing schemes for sampling" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Eurachem_Proficiency_testing_schemes_for_sampling_3_73f9231ef6.jpg 104w," sizes="100vw" width="578" height="862"><h4>How to apply ISO/IEC 17043 to sampling PT&nbsp;</h4>The following requirements from ISO/IEC 17043 might need some particular consideration for sampling PT:&nbsp;<ul><li>Personnel: The demonstration of the competence (knowledge of the planning of sampling, sampling techniques and preparation of sampling sites) of the personnel involved in organizing the sampling PT scheme.&nbsp;</li><li>Equipment, accommodation and environment: Environmental conditions should be taken into consideration by including them in the performance evaluation or by minimising or eliminating their influence.&nbsp;</li><li>Planning: Production, quality control, storage and distribution of proficiency test items for sampling PTs can be interpreted as “requirements for the sampling site” and handling/transportation of the samples once the sampling is performed.&nbsp;</li><li>Preparation of PT items: The sampling site must be prepared to ensure that each participant performing the sampling has an equivalent challenge (possible influences: rain, wind, temperature, participant, etc.).&nbsp;</li><li>Homogeneity and Stability: The item that is being sampled should be as similar as possible for all participants during the sampling exercise. Special care should be taken to minimise the influence of any previous participants in the exercise, for example by causing drill holes. Dynamic systems such as a river by their nature are constantly changing and therefore may not be homogeneous or stable.</li><li>Statistical design: ISO 13528 [2] should be considered when establishing the statistical design. It is important to distinguish between the sampling procedure and the test method in the statistical design, which will depend on the type of sampling PT. Sample transport effects that could have an influence should also be considered.&nbsp;</li><li>Assigned value / Evaluation criteria: The determination of any assigned value will depend on the type of sampling PT. The evaluation criteria should also consider the pre-sampling (e.g. container used) and the post-sampling (e.g. sample storage and transportation) aspects.&nbsp;</li><li>PT items handling and storage: It should be considered that the PT item includes both the sampling site and the samples taken during the sampling activity.&nbsp;</li><li>Packaging, labelling and distribution of PT items: The PT provider should provide clear instructions when specific labelling and packaging is required. Where there is a direct measurement the requirements of this section are not applicable.&nbsp;</li><li>Data analysis and records: Where the performance evaluation is based on a comparison to a reference procedure, this can be purely qualitative. Alternatively, observed deviations can be converted to numerical scores (e.g. 0 for negligible, 1 for minor, 2 for major) and some appropriate statistical data analysis performed.&nbsp;</li><li>Confidentiality and Collusion: When all the participants (or groups) are performing the sampling at the same time, then this must be made clear to the participants since confidentiality cannot be ensured and reasonable precautions need to be taken to prevent collusion given this situation.</li></ul><img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Eurachem_Proficiency_testing_schemes_for_sampling_2_d50329a4b4.jpg" alt="Eurachem: Proficiency testing schemes for sampling" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Eurachem_Proficiency_testing_schemes_for_sampling_2_d50329a4b4.jpg 210w," sizes="100vw" width="1099" height="816"><h4>Conclusion&nbsp;</h4>PT schemes for sampling play an important role in the improvement of sampling procedures and also in the development of participants from an educational point of view, especially if workshops with the participants are organised. PT schemes for sampling can also give an appreciation of the contribution of the sampling in relation to the overall quality of the measurement, including the associated contribution to the measurement uncertainty.&nbsp;<h4>More information and further reading&nbsp;</h4>Information about PT providers and schemes can be obtained from your national accreditation body, from the <a target="_blank" rel="noopener noreferrer" href="https://www.eptis.org/">EPTIS website</a> or from other national or international organizations.&nbsp;* <a target="_blank" rel="noopener noreferrer" href="https://pubs.rsc.org/en/content/articlelanding/2017/ay/c7ay90092a">Proficiency testing of sampling. AMC Technical Brief 78, 2017</a>Eurachem Guides/Leaflets:&nbsp;<ul><li>Selection, Use and Interpretation of Proficiency Testing Schemes by Laboratories, 2nd edition, 2011&nbsp;</li><li>Measurement uncertainty arising from sampling, 2nd edition, 2019&nbsp;</li><li>Leaflet on Pre- and post-analytical proficiency testing, 1st edition, 2009</li></ul>

Proficiency testing schemes for sampling

Test items are sometimes available from proficiency testing (PT) providers after the completion of a PT round. The purpose of this information leaflet is to advise laboratories on benefits and limitations of surplus PT items.&nbsp;<h4>Potential uses&nbsp;</h4>Surplus PT items have a number of potential uses including:&nbsp;<ul><li>Assessing new measurement procedures and verifying their correct implementation;&nbsp;</li><li>Training of new analysts;&nbsp;</li><li>Assessing the likely performance in a PT scheme;&nbsp;</li><li>Troubleshooting of measurement procedures and reassessing analytical performance following questionable or unacceptable results reported in a PT round;&nbsp;</li><li>As internal quality control (IQC) samples [1].&nbsp;</li></ul><h4>General considerations&nbsp;</h4>The following factors should be considered prior to use of surplus PT items:<ul><li>Check whether any relevant accompanying information is available and meets the end-user requirements, e.g. the assigned value and associated uncertainty of the measurand(s) of interest.&nbsp;</li><li>Assess their physical suitability and ensure that the matrix is appropriate. Some PT items have compositions close to that of routine samples, while others are synthetic or fortified (spiked). This may influence the suitability of the PT item for assessing different stages of the measurement procedure.&nbsp;</li><li>Check their availability. Surplus PT items may only be available in small numbers which will limit their usefulness for repeated testing over time, for example, as IQC samples.&nbsp;</li><li>Assess the stability information provided. PT items must be stable during the conduct of a round but there is no requirement for PT items to be assessed for stability following the completion of the round. The PT provider may be able to provide additional stability data or storage requirements, otherwise further evaluations by the end-user will be necessary.&nbsp;</li></ul><h4>Usefulness of accompanying PT data&nbsp;</h4>At the end of a PT round an assigned value is provided for each parameter, and participants’ performances are judged against a predefined performance acceptability criterion, for example, using a z score [2]. This information can support the use of surplus test items provided the following points are considered:&nbsp;<ul><li>If the assigned value is a consensus value end-users may need to reflect on the measurement procedure used to derive it;&nbsp;</li><li>Whether the metrological traceability of the assigned values makes them suitable for assessing measurement bias, for example if derived from a known spike value or a measurement made with a primary measurement procedure;</li><li>Whether the PT provider’s performance evaluation criteria agree with the fitness-for-purpose criteria required by the end-user. If so they could be used to set control limits in IQC charts. Otherwise own criteria have to be set [3].&nbsp;</li></ul><h4>Case study - Use of surplus PT items when implementing a standard method&nbsp;</h4>A laboratory wishes to adopt the standard method EN 15763 for the determination of the mass fraction of cadmium in foodstuffs by ICP-MS after microwave digestion. To this purpose, five PT items with different matrices and levels (mass fraction) were obtained from a PT provider and analysed in duplicate.&nbsp;The laboratory (i) verifies that the average of the replicates falls within the satisfactory performance range for the respective PT rounds (| z score | ≤ 2) and (ii) checks that the observed difference of the replicates does not exceed the repeatability limit (r) of the standard method.&nbsp;The data below confirms that the laboratory can operate the method correctly.<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Eurachem_Use_of_surplus_proficiency_test_items_case_study_8e190bd7d7.jpg" alt="Eurachem: Case study - Use of surplus PT items when implementing a standard method " srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Eurachem_Use_of_surplus_proficiency_test_items_case_study_8e190bd7d7.jpg 245w," sizes="100vw" width="1600" height="359">

Use of surplus proficiency test items

A laboratory will occasionally have a poor performance in a PT scheme. When this occurs, the laboratory should acknowledge it, carry out an investigation and document a review of possible causes, even if it decides not to take any specific action. The purpose of this leaflet is to advise laboratories on how to best address such events. A good management of the poor performance in a PT can save you time and money.&nbsp;<h4>Evaluation of poor performance&nbsp;</h4>Every unsatisfactory performance score indicates a problem which needs to be investigated. In addition the laboratory should set its own criteria for launching an investigation with regards to other indicators of possible poor performance, such as questionable performance scores or observed trends.<h4>Root cause investigation&nbsp;</h4>The depth of the investigation will depend on several factors, such as the importance of the analysis, the frequency of unsatisfactory results and evidence of trends.&nbsp;The laboratory should check whether the PT report gives an explanation for the unsatisfactory performance. If no reason is provided, a stepwise approach is preferable to maximize the chances of determining the root cause of the problem. This approach is described in Annex B of the Eurachem Guide [1].<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Eurachem_How_to_investigate_poor_performance_in_proficiency_testing_Annex_B_be140d5b39.jpg" alt="Eurachem: How to investigate poor performance in proficiency testing - Annex B" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Eurachem_How_to_investigate_poor_performance_in_proficiency_testing_Annex_B_be140d5b39.jpg 153w," sizes="100vw" width="726" height="740">The investigation should consist of the following steps and involve the personnel that performed the analysis and where approporiate, also involve the laboratory management:&nbsp;<ol><li>Analyse the raw data, internal quality control data, any trend from previous PT rounds and the overall performance of the participants in that round;&nbsp;</li><li>After investigation is complete, make a plan for corrective action and consider implications for previous test results;&nbsp;</li><li>Execute and record the corrective action(s);&nbsp;</li><li>Check whether the corrective action(s) was effective.&nbsp;</li></ol><h4>Causes for poor performance&nbsp;</h4>The causes of poor performance can be grouped into three categories:&nbsp;<ol><li>Clerical errors are not directly linked to the laboratory’s technical competence, but can underline that the laboratory may have a potential problem when reporting results to its customers. Clerical errors can include transcription errors, mislabelling, incorrect units and decimal point errors. Identifying them is an important first step of an investigation. If such errors are a regular cause of poor performance, the investigation should focus on the related aspects of the quality management system.&nbsp;</li><li>Technical problems can occur at any step of the analytical procedure. If the investigation does not enable the laboratory to identify the root cause, it may be necessary to review the method validation.&nbsp;</li><li>Problems related to the PT scheme. Poor performance could also be due to the PT scheme not being entirely appropriate. The Eurachem Guide [1] provides information on selecting an appropriate PT scheme. In other cases, a problem may have occurred with the PT items. The laboratory is encouraged to discuss its findings with the PT provider.&nbsp;</li></ol>The “Fishbone diagram” or “5 Whys” are useful tools to investigate the root cause of a problem.<h4>Case studies&nbsp;</h4><h5>Example 1</h5>A medical laboratory has provided results in a PT scheme for tumor markers in serum with a 20 % deviation from the assigned value, although the daily internal controls performed well. The laboratory investigates the process and discovers the mistake was due to an incorrect unit conversion factor being introduced for PT results when the IT system was last updated. Since patient results are reported in a different unit than the one used by the PT provider no patient results were affected.&nbsp;<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Eurachem_How_to_investigate_poor_performance_in_proficiency_testing_0df20b3875.jpg" alt="Eurachem: How to investigate poor performance in proficiency testing" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Eurachem_How_to_investigate_poor_performance_in_proficiency_testing_0df20b3875.jpg 245w," sizes="100vw" width="1539" height="815"><h5>Example 2</h5>A laboratory performing food and feed analyses obtained poor performance scores on Listeria monocytogenes enumeration, because the number of microorganisms found was underestimated. Laboratory investigations showed an issue in incubation time and temperature applied by the analyst. The laboratory found that the analyst used an out-of-date procedure.<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Eurachem_How_to_investigate_poor_performance_in_proficiency_testing_2_4e56101bbb.jpg" alt="Eurachem: How to investigate poor performance in proficiency testing" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Eurachem_How_to_investigate_poor_performance_in_proficiency_testing_2_4e56101bbb.jpg 245w," sizes="100vw" width="1185" height="526">

How to investigate poor performance in proficiency testing

The Eurachem Guide on “Selection, Use and Interpretation of Proficiency Testing (PT) Schemes” [1] recommends participants to consider the statistical approach used by the PT provider when selecting a PT scheme. This leaflet is intended to help participants in quantitative PT schemes to better understand the statistical parameters that describe the data distribution in the PT report. The PT report should also provide information on how the assigned value (xpt), the corresponding uncertainty (u (xpt)) and the standard deviation for proficiency assessment (σpt) were obtained [2].&nbsp;Summary statistics reported in PT schemes typically describe two features of a data set or of an underlying normal distribution of technically valid results. One feature is the location, usually described by some kind of average. The second is the dispersion, or spread of the data. A further important summary statistic is the (standard) uncertainty associated with the location, which can help to determine whether the chosen location estimator differs from (for example) an external reference value or from individual reported results. The different statistics used for these purposes are summarised below.&nbsp;<h4>Location of the results reported by participants&nbsp;</h4>An estimated location is usually reported even if it is not used as the assigned value. ISO 13528 [3] describes different ways to estimate the location using the arithmetic mean or median, or applying sophisticated robust statistical methods (Table 1).&nbsp;<h5>Key points to note:&nbsp;</h5><ul><li>When data sets contain outliers or when symmetry of the data set cannot be verified, ISO 13528 strongly recommends to use robust estimators.&nbsp;</li><li>When using the arithmetic mean, the PT provider would normally have checked the data set and removed any outliers.&nbsp;</li><li>All estimates of location can be unreliable for data sets with a small number of results.&nbsp;</li><li>A significant difference between the estimated location and a reference value may indicate a bias of the analytical methods (maybe just one, some or for all methods) that have been used by the participants.&nbsp;</li><li>If the participants used different analytical methods, the PT provider might report different estimated locations for each of these methods.&nbsp;</li></ul><h4>Dispersion of results reported by participants&nbsp;</h4>The standard deviation is an estimate of dispersion of a data set. There are various possibilities to estimate the despersion of data including MADe, nIQR, Algorithm A, Qn or Q method (Table 1). Often, they are normalised to allow direct comparability to the standard deviation of a normally distributed data set.&nbsp;<h5>Key points to note:&nbsp;</h5><ul><li>The dispersion is expressed in the same units as the measurand or reported as a percentage.&nbsp;</li><li>When estimating the classical standard deviation, the PT provider would normally have checked the data set and removed any outliers.&nbsp;</li><li>All estimates of dispersion can be unreliable for data sets with a small number of results.</li></ul><img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Eurachem_Table_1_Pros_and_cons_of_the_various_statistical_approaches_89d477c289.jpg" alt="Eurachem: Table 1 - Pros &amp; cons of the various statistical approaches" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Eurachem_Table_1_Pros_and_cons_of_the_various_statistical_approaches_89d477c289.jpg 245w," sizes="100vw" width="1600" height="911"><h4>Standard uncertainty of the location&nbsp;</h4>The standard uncertainty of the location is a measure of the reliability of the mean that can be important for participants.&nbsp;<h5>Key points to note:&nbsp;</h5><ul><li>The standard uncertainty of the arithmetic mean is calculated as:<ul><li>&nbsp;<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Snimek_obrazovky_2026_06_04_195552_9d4102aaef.png" alt="" width="226" height="70"></li><li>where s is the classical standard deviation, and p the number of reported results.&nbsp;</li></ul></li><li>For other estimators of the mean ISO 13528 suggests to use an additional (conservative) multiplication factor set to 1.25, hence:<ul><li><img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Snimek_obrazovky_2026_06_04_195606_d264be5dd8.png" alt="" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Snimek_obrazovky_2026_06_04_195606_d264be5dd8.png 245w," sizes="100vw" width="301" height="49"></li><li>where s* is the robust standard deviation from MADe, nIQR, Algorithm A, Qn or Q method, and p the number of reported results.</li></ul></li></ul>

Understanding PT statistics

This leaflet is intended to help participants in quantitative proficiency testing (PT) schemes to better understand the performance assessment made by the PT provider [1-4].<h3>Performance assessment parameters&nbsp;</h3><h4>Assigned value&nbsp;</h4>In order to assess individual performances, the results reported by participants are compared to the value assigned by the PT provider (xpt). ISO 13528 [1] suggests five different ways to obtain xpt (Figure 1). In this context, the design of the PT scheme must reflect the objectives of the scheme, comparing results either to a predetermined xpt (independent from the participant results), or to a value derived from the reported results. The PT provider is responsible for defining xpt , taking into account the advantages and disadvantages of the various possibilities when establishing metrological traceability or estimating the uncertainty of the assigned value, u(xpt).<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Eurachem_Figure_1_e5ef5d1f9c.jpg" alt="Eurachem - Figure 1 Options for obtaining the assigned value, Xpt" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Eurachem_Figure_1_e5ef5d1f9c.jpg 245w," sizes="100vw" width="640" height="382"><h4>Uncertainty of the assigned value&nbsp;</h4>The uncertainty of the assigned value u(xpt) can be estimated in at least five different ways [1], each corresponding to the value assignment strategy illustrated in Figure 1. The different uncertainty evaluation methods are listed in Figure 2. This uncertainty can be reported as a standard uncertainty (u (xpt)), or as an expanded uncertainty U (xpt) with a coverage factor k, where U (xpt) = k . u (xpt). The choice of k will impact the level of confidence (e.g. approximately 95 % for k = 2).<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Eurachem_Figure_2_f41ebc0c61.jpg" alt="Eurachem: Figure 2 Options for obtaining the uncertainity of the assigned value, u(Xpt)" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Eurachem_Figure_2_f41ebc0c61.jpg 245w," sizes="100vw" width="727" height="378"><h4>Standard deviation for proficiency assessment&nbsp;</h4>Five possibilities are suggested by ISO 13528 to set the standard deviation for proficiency assessment spt (Figure 3). The chosen approach must comply with the objectives of the PT scheme.&nbsp;<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Eurachem_Figure_3_f09cb3969e.jpg" alt="Eurachem: Figure 3 Options for obtaining the standard deviation for proficiency assesment" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Eurachem_Figure_3_f09cb3969e.jpg 245w," sizes="100vw" width="638" height="355">Two approaches for performance assessments use spt as shown in Table 1.<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Eurachem_Table_1_Scoring_for_performance_assessment_3dd03aa3fb.jpg" alt="Eurachem: Table 1 - Scoring for performance assessment." srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Eurachem_Table_1_Scoring_for_performance_assessment_3dd03aa3fb.jpg 245w," sizes="100vw" width="1390" height="571"><h3>Performance assessment&nbsp;</h3>All performance assessments are based on the difference between the participant’s results (xi) and the assigned value (xpt) divided by a normalisation factor (Table 1).&nbsp;<ul><li>The percent difference D% is the normalised difference expressed as a percentage of xpt , for which a prescribed maximum relative permissible error (dE% = dE / xpt) is defined by the PT provider.&nbsp;</li><li>The z score (unitless) compares the participant’s deviation from the assigned value with the standard deviation for proficiency assessment (spt). When the uncertainty of the assigned value is significant (u(xpt) &gt; 0.3 spt), the z’ score should be used instead, to include the contribution of u(xpt) in the denominator.&nbsp;</li><li>The zeta (ζ) score (unitless) states whether the laboratory’s result agrees with the assigned value within the respective uncertainty. The denominator is the combined (standard) uncertainty of the assigned value u(xpt) and the (standard) measurement uncertainty reported by the laboratory u(xi). An unsatisfactory zeta score can either be caused by an inappropriate estimation of the measured quantity value (e.g. concentration, content), or of its measurement uncertainty, or both.&nbsp;</li><li>The En score (unitless), unlike the zeta score, uses a combination of expanded uncertainties as the normalisation factor. This combination corresponds to a confidence interval of approximately 95 %. En scores are often used in metrology comparisons among calibration laboratories.</li></ul><h3>Interpretation of performance scores&nbsp;</h3>According to ISO 13528:2022 clause 9.4.2, the conventional interpretation of the z, z’ and ζ scores is as follows:&nbsp;<ul><li>A result that gives |score| ≤ 2.0 is considered to be acceptable, indicating satisfactory performance.&nbsp;</li><li>A result that gives 2.0 &lt; |score| &lt; 3.0 is considered to give a warning signal, indicating questionable performance.&nbsp;</li><li>A result that gives |score| ≥ 3.0 is considered to be unacceptable (or action signal), indicating unsatisfactory performance.&nbsp;</li></ul>Similarly, |En| &lt; 1 and |D%| &lt; dE% indicate successful (acceptable) performance.

Understanding PT performance assessment

Interlaboratory comparisons (ILCs) have been used for more than a century and many important concepts are linked to them. This leaflet addresses the basic terminology of ILCs, why some colloquial terms can be misleading or misunderstood, and the importance of harmonisation.&nbsp;An ‘ILC’ is the organisation, performance and evaluation of measurements or tests on the same or similar items by two or more laboratories in accordance with predetermined conditions [1]. ILCs have a variety of overall – and detailed – objectives, the three most common [2] being:<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Eurachem_Let_s_call_a_PT_scheme_a_PT_scheme_3_9993b9fb0f.jpg" alt="Eurachem: Let’s call a PT scheme a PT scheme" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Eurachem_Let_s_call_a_PT_scheme_a_PT_scheme_3_9993b9fb0f.jpg 245w," sizes="100vw" width="1516" height="472"><h4>Some comparisons have special names&nbsp;</h4>The evaluation of participant (laboratory, organisation or individual) performance is typically referred to as ‘proficiency testing’ (PT) or ‘external quality assessment’ [1]. When the main objective is evaluating the performance of, for example, a candidate standard method, the ILC is often called a ‘method performance study’ or a ‘collaborative study’ [2, 3]. The former is quite clear whereas wording such as “collaborative” and “cooperative” just stresses joint effort. A project aimed at producing a new certified reference material may involve several laboratories conducting analyses on agreed properties [4]. Not surprisingly, this work is sometimes labelled as a ‘material certification study’. Special ILCs used to demonstrate measurement capabilities of National Metrology Institutes are called ‘key comparisons’, ‘supplementary comparisons’, and ‘pilot studies’ [5].&nbsp;<h4>Some terms can be confusing&nbsp;</h4>Other terms for ILCs, including ‘ring test’, ‘round robin’ and ‘circle analysis’, are common in the literature. They have, however, been used for different objectives and different designs and may be ambiguous or understood differently by laboratory staff. The use of these terms may, for example, imply a specific way of distributing the ILC sample. ISO/IEC 17043 uses ‘sequential scheme’, for a PT where a unique item is transferred between the participants and ‘simultaneous scheme’, in which similar items from a batch are distributed simultaneously to the participants [1].&nbsp;Terms like ‘measurement audit’, ‘interlaboratory testing scheme’, ‘measurement comparison scheme’, ‘rapid performance evaluation scheme’ and ‘intercalibration’ may not correctly reflect the category of laboratory involved, or the type of work participants do during the ILC.&nbsp;When in 1994 the IUPAC defined and described three major types of ‘interlaboratory studies’, the word ‘study’ was preferred to synonyms such as ‘trial’, ‘exercise’, ‘test’, ‘evaluation’ or ‘check’ [2].<h4>Let’s try to harmonise&nbsp;</h4>Many laboratory staff are unaware of official terms related to ILCs but recognise the acronym PT and words like ‘ring test’. Ideally, we would use a common scientific language, but harmonisation is time consuming. Terms appear and disappear and definitions change. The same term can be defined differently, even in different international standards. Lack of standards and guidelines in local languages can cause confusion and contribute to spreading of less appropriate terms.&nbsp;<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Eurachem_Let_s_call_a_PT_scheme_a_PT_scheme_2_820043ce8b.jpg" alt="Eurachem: Let’s call a PT scheme a PT scheme" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Eurachem_Let_s_call_a_PT_scheme_a_PT_scheme_2_820043ce8b.jpg 234w," sizes="100vw" width="1006" height="671">Much confusion can be avoided by clear reference to relevant standards and sectoral guides. ISO/ IEC 17043 [1] and ISO 13528 [6] contain many important concepts related to PT schemes and other ILCs. Some of these are also incorporated into ISO/IEC 17025 [7]. Additional useful, and free, tools are the Eurachem PT Guide [8] and the ISO online browsing platform [9].&nbsp;In writing guidance and instructions, remember that the term ‘interlaboratory comparison’ has a broad meaning and can be used for all cases described above. Sometimes this is clear enough for the reader. In other cases, use acknowledged terms that reflect the detailed objective of the ILC. Let’s call a PT scheme a PT scheme!

Let’s call a PT scheme a PT scheme!

The international standard ISO/IEC 17025 [1] clause 7.7.2 with regards to ensuring the validity of results, requires a laboratory to participate in proficiency testing (PT) and/or to participate in interlaboratory comparisons (ILCs) other than PT. However, no guidance as to what these other ILCs might be, or indeed how they might compare in relation to PT, is provided. This leaflet aims to outline other ILCs available, pointing out some of the limitations as compared to PT.&nbsp;There are three major types of ILCs in which a group of laboratories analyses identical portions from a homogeneous, stable test sample, with each type being characterized by its intended purpose [2,3]:<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Eurachem_Interlaboratory_comparisons_other_than_proficiency_testing_2_d9263807fb.jpg" alt="Eurachem: Interlaboratory comparisons other than proficiency testing" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Eurachem_Interlaboratory_comparisons_other_than_proficiency_testing_2_d9263807fb.jpg 245w," sizes="100vw" width="1507" height="607">A key point is that PT is an ILC that is designed to specifically evaluate the performance of participants, providing them with an infrastructure to monitor and assess the validity of their results. This is not the key purpose of the two other ILCs, and thus depending on their intended purpose they have a number of limitations that laboratories need to be aware of when using them rather than PT. Only proficiency testing should have been conducted in full compliance to the competency standard ISO/IEC 17043 [4].&nbsp;<h4>Evaluation of measurement procedure performance&nbsp;</h4>These ILCs, are designed to assess how a particular measurement procedure performs and whether it is fit for its intended purpose. These ILCs are often referred to as a ‘method performance study’ or a ‘collaborative study’ [2]. Some limitations of using such ILCs for monitoring laboratory performance as an alternative to PT are:&nbsp;<ul><li>All participants use the same measurement procedure i.e. the measurement procedure that is being validated. Unless the measurement procedure is being routinely used by the participating laboratory such an ILC will not assess the routine performance of the laboratory using their own routine measurement procedure. Nor does it enable them to compare their performance with other laboratories using alternative measurement procedures.&nbsp;</li><li>The measurement procedure is likely to be new, so participants may not have established stable procedures and quality control, so performance may not reflect routine use.&nbsp;</li><li>The report from such an ILC would not provide information on the performance of the participant, if the statistical model used makes the assumption that all laboratories perform with equal variability.</li><li>The design of the study may require the participants to assess multiple samples and/or to undertake multiple replicate analysis on the test materials. This may not be representative of the routine measurements or tests undertaken by the laboratory.&nbsp;</li><li>The conclusion of the ILC might be that the measurement procedure being assessed is not valid.&nbsp;</li></ul><img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Eurachem_Interlaboratory_comparisons_other_than_proficiency_testing_3_a53ecd2f5d.jpg" alt="Eurachem: Interlaboratory comparisons other than proficiency testing" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Eurachem_Interlaboratory_comparisons_other_than_proficiency_testing_3_a53ecd2f5d.jpg 157w," sizes="100vw" width="570" height="566"><h4>Characterisation of candidate reference materials&nbsp;</h4>These are ILCs that assign a property value to a candidate reference material, usually with a stated measurement uncertainty. These are often referred to as a ‘material certification study’ [2]. Limitations of using such ILCs as an alternative to PT are:&nbsp;<ul><li>The analysis of the candidate reference material is likely to require additional care in exercising the routine measurement procedure, such as substantially more replication, and/or may require quite different reporting formats (e.g. all valid replicates instead of a mean result). Thus, such an ILC will not necessarily provide an evaluation of the performance of the laboratory based on the routine use of their measurement procedures.&nbsp;</li><li>The report from such an ILC may include the results from the participating laboratories but would not provide information on the performance of each laboratory.&nbsp;</li><li>Participating laboratories may be required to assess multiple samples and/or to undertake multiple replicate analysis on the candidate reference materials. Thus this may not be representative of the routine measurements or tests undertaken by the laboratory.&nbsp;</li></ul><h4>Other ILCs&nbsp;</h4>Some other ILCs, whilst not being conducted in full compliance of the requirements of ISO/IEC 17043, may be designed to evaluate participant performance. These would include small ILCs as defined by EA‑2/21 [5] or split sample ILCs i.e. where a portion of a single sample is sent to two or three laboratories. Whilst these ILCs are specifically designed to evaluate participant performance, they may not provide a comprehensive comparison that a PT might provide. In such cases the laboratory needs to evaluate carefully the competence of the organization managing the ILC and the suitability of the service externally provided (ISO/IEC 17025 clause 6.6).

Interlaboratory comparisons other than proficiency testing

<h4>Do you use results of chemical analyses as a basis for your decisions and judgements?</h4>Those of us working in accredited laboratories or dealing with issues concerning the quality of measurements, would like to inform you about some important changes concerning the way the results of measurements are presented. These changes make it easier for you as an end-user to make correct decisions.<h4>Nobody is perfect!</h4>Results of analyses cannot be perfect! We hope this does not come as a big surprise to you. We use the term measurement uncertainty to describe this lack of perfection.<h4>The analytical process</h4>In each step of the analytical work, from sampling to the final measurement, deviations from the true value occur and measurement conditions vary. We take measures and perform controls regularly to assure that these deviations and variations together are small enough to make sure the end result fulfils your requirements. When we don’t have full information about all of the steps, e.g. when sampling and initial sample preparation are performed by you as a customer, you can assist us by providing detailed information about how that work was performed. Our experts are ready to advise on all matters regarding sampling. Please contact the laboratory beforehand.<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Eurachem_Important_information_to_our_customers_concerning_the_quality_of_measurements_2_d885f6b051.jpg" alt="Eurachem: The analytical process" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Eurachem_Important_information_to_our_customers_concerning_the_quality_of_measurements_2_d885f6b051.jpg 139w," sizes="100vw" width="562" height="630"><h4>Results should be fit-for-the-purpose</h4>The accuracy of the results must of course not be too low nor too high since this would increase the costs. It should be fit for the intended purpose. If you are unsure on what level of accuracy you need, do not hesitate to contact the laboratory.<h4>Uncertainty and limiting values</h4>Many analyses are made to assure that limiting values are not exceeded. Without information about the measurement uncertainty it may appear to be very easy to make decisions, but these decision may be incorrect, with, e.g. economical consequences when rejecting instead of accepting a product, judicial consequences when returning a verdict of guilty instead of not guilty, or medical consequences when carrying out an unnecessary treatment. There are numerous examples!<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Eurachem_Important_information_to_our_customers_concerning_the_quality_of_measurements_3_f0ef8edc77.jpg" alt="Eurachem: Uncertainty and limiting values" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Eurachem_Important_information_to_our_customers_concerning_the_quality_of_measurements_3_f0ef8edc77.jpg 245w," sizes="100vw" width="932" height="453">With a realistic measurement uncertainty the information included in the result becomes much more useful.<h4>It will be easier to compare results</h4>Most laboratories have until now chosen not to state measurement uncertainty in the test report. Instead, such information has only been given when the customer has asked for itIn the future, information about the measurement uncertainty will appear more frequently in the test report. It is also possible that you will bump into new and unfamiliar quality terms. This is due to the fact that there are new international guides and standards describing a common and partly new terminology. One of the objectives is to make it easier for you as a customer to compare test results.<h4>What could it look like?</h4>When reporting the test result we will give the normal information about what we have measured. When the results are followed by uncertainty statements, they are presented as intervals within which the true values are expected to lie with a certain level of confidence (usually 95%). In the example below the lead content is 1.65 ± 0.15 mmol·kg-1, that is between 1.50 and 1.80. The measurement uncertainty is also often reported relatively, in %.<ul><li>Total lead content (Pb): 1.65 mmol·kg-1&nbsp;</li><li>Measurement uncertainty: 0.15 mmol·kg-1 (9.1%)&nbsp;</li></ul>The stated uncertainty is an expanded measurement uncertainty (U). It was obtained by multiplying the combined standard uncertainty uc with a coverage factor k equal to 2. This corresponds approximately to a 95 % confidence interval.<h4>All’s well that ends well...</h4>The requirements for a consistent way of reporting test results are increasing. Therefore, those of us involved in measurements are eager to assure ourselves that we understand your needs. You will notice this in your contacts with us before, during and after the test assignment. We hope that you will be satisfied with the final result.<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Eurachem_Important_information_to_our_customers_concerning_the_quality_of_measurements_4_f870b1a7b8.jpg" alt="Eurachem: All’s well that ends well..." srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Eurachem_Important_information_to_our_customers_concerning_the_quality_of_measurements_4_f870b1a7b8.jpg 159w," sizes="100vw" width="713" height="701">

Important information to our customers concerning the quality of measurements

<h4>Traceability in the 19th century&nbsp;</h4>Once a dairymaid bought one kilo of flour from the local shop. Next day she returned to sell one kilo of butter to the shopkeeper. He then complained that 50 grams of the kilo were missing.&nbsp;<blockquote>Oh that is odd, the dairymaid said, to get the correct weight I used the kilo of flour you sold to me yesterday to weigh the butter!&nbsp;</blockquote><h4>Traceability today&nbsp;</h4>Traceability is the ability to trace, for example, the ancestors of a person or the origin of a product. The word can be derived from the Latin tractus = drawn or the verb trahere = to draw.&nbsp;Traceability can refer to documentation (e.g. a sampling procedure), a laboratory, analyst or method. But as in ISO/IEC 17025, this leaflet deals with traceability of measurement results. That is, measurement results have to be traceable to the metrological1 references used. To be specific, we use the term metrological traceability.&nbsp;<blockquote>The current VIM2 definition of metrological traceability is: property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations each contributing to the measurement uncertainty2&nbsp;</blockquote><h4>The references&nbsp;</h4>Ideally the references should be the values of national and international standards expressed in SI units. The traceability is realised through chains of calibrations, the traceability chain. For temperature and many other physical quantities such as mass and time, the traceability is easily established.&nbsp;In addition, in chemistry the working standards are substances with defined purity, solutions of pure substances and matrix reference materials.<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Eurachem_Metrological_Traceability_of_Analytical_Results_2_b809be57bc.jpg" alt="Douglas Hasbun: An “analyst” not knowing the traceability of his/her values for volume, time, mass, calibration solution concentration and temperature." srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Eurachem_Metrological_Traceability_of_Analytical_Results_2_b809be57bc.jpg 157w," sizes="100vw" width="852" height="847"><h4>Temperature example - Traceability to SI&nbsp;</h4>The temperature of a sample can be traced back through a chain of calibrations to the reference, an SI traceable value of temperature at 0 °C.<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Eurachem_Metrological_Traceability_of_Analytical_Results_3_9933347331.jpg" alt="Douglas Hasbun: Temperature example - Traceability to SI" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Eurachem_Metrological_Traceability_of_Analytical_Results_3_9933347331.jpg 245w," sizes="100vw" width="926" height="432"><h4>Example – Mercury in tuna fish&nbsp;</h4>A measurement result (mass fraction) obtained for mercury in a tuna fish sample is 4.03 ± 0.11 mg/kg. The result is reported as total Hg on dry weight basis (105 ºC, 12 h determined on a separate sample portion) and the measurement uncertainty is reported with a 95 % level of confidence (coverage factor, k=2). In this case mercury was determined with a mercury analyser (cold vapour atomic spectroscopy) after a microwave digestion.&nbsp;The samples are weighed on a balance with a calibration certificate relating the weight measured to the SI unit of mass (kg).&nbsp;The acid digest is diluted in a volumetric flask where the manufacturer documents the traceability of the volume of the flask to a national standard.&nbsp;The calibration curve was prepared using a CRM (certified reference material) – a mercury standard with a certificate stating a quantity value of 0.998 ± 0.005 mg/L (k=2) and with traceability to pure mercury.&nbsp;The method is validated using an appropriate matrix CRM with a total mercury mass fraction of 1.97 ± 0.04 mg/kg (k=2). The purpose of the validation is to check the method performance.<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Eurachem_Metrological_Traceability_of_Analytical_Results_4_1ef2bc4b97.jpg" alt="Douglas Hasbun: An “analyst” knowing the traceability of his/her values for volume, time, mass, calibration solution concentration and temperature" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Eurachem_Metrological_Traceability_of_Analytical_Results_4_1ef2bc4b97.jpg 119w," sizes="100vw" width="808" height="1062"><h4>How can a laboratory demonstrate metrological traceability?&nbsp;</h4>The evidence required by the laboratory to demonstrate traceability for the mercury result is shown below:&nbsp;<ol><li>concentration of the Hg solution – a certificate for the CRM solution;&nbsp;</li><li>mass of sample – calibration certificate for the balance;&nbsp;</li><li>volume of volumetric flask – calibration certificate of the manufacturer;&nbsp;</li><li>drying temperature – calibration of oven temperature;&nbsp;</li><li>digestion conditions – calibration of e.g. temperature;&nbsp;</li><li>drying time – ordinary clock or stopwatch&nbsp;</li></ol>Point 1 would need special attention to assure the quality and traceability for the calibration standard. Traceability for points 2, 3 and 6 is easily achieved with adequate uncertainty using commercial equipment.&nbsp;Points 4 and 5 need additional attention by the lab.&nbsp;The use of the matrix CRM in the method validation is vital but not part of the traceability chain, since it is not used for calibration. If this CRM is used for recovery correction it should be part of the traceability chain.&nbsp;<h4>Measurement quality&nbsp;</h4><ul><li>Method validation demonstrates that the method (set of conditions) used in a particular laboratory at a certain time was fit for purpose and all significant effects were identified.&nbsp;</li><li>Calibration of critical equipment completes the metrological traceability chains.</li><li>Measurement uncertainty is estimated from the method validation and the traceability.&nbsp;</li><li>Quality control (internal and external) assures that the measurement results (including uncertainty) are of the same quality as at the time of validation.</li></ul><h4>Conclusion</h4>The traceability of a measurement result refers to metrological traceability as defined by VIM. It relates the result to SI units or other agreed standards/references.&nbsp;Traceability is essential for comparability of results and is a requirement of ISO/IEC 17025. Traceability is easily achieved following good laboratory practice.

Metrological Traceability of Analytical Results

When test results are used to assess compliance, i.e. to decide whether specifications or regulations are met, the measurement uncertainty of the test results has to be taken into account. Assessment of compliance for cases i and v in the Figure below is clear – the measurement results including the uncertainty interval are clearly above or below the limit value. In the other cases the decision is not clear since the uncertainty interval overlaps the limit value. The Eurachem/CITAC guide [1] gives guidance on cases ii, iii and iv.&nbsp;<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Eurachem_Upper_control_limit_9366d8f597.jpg" alt="Eurachem: Upper control limit" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Eurachem_Upper_control_limit_9366d8f597.jpg 245w," sizes="100vw" width="1536" height="585"><h4>We need acceptance &amp; rejection zones&nbsp;</h4>In order to judge whether the results in cases ii, iii, and iv comply with the limit value we need a decision rule, based on the risks associated with making a wrong decision. This decision rule enables a guard band, g, to be calculated which defines an acceptance zone and a rejection zone. If the measurement value is within the acceptance zone the specifications are met and we can conclude compliance. If the measured value is in the rejection zone, we can conclude non-compliance. The intersection between these two zones is called the decision limit. A guard band is normally chosen so that for a measured value in the acceptance zone the probability of correct acceptance is more than or equal to a defined confidence value α. Note that a guard band, g = 0 can also be used. This is called simple acceptance.&nbsp;<h4>Decision rule&nbsp;</h4>A decision rule should have a well-documented method of determining the location of acceptance and rejection zones, ideally including acceptable levels of probability, P, that the value of the measurand 1) lies within the specification limit, high confidence of correct acceptance (low probability of false acceptance) or 2) lies outside the specification limit, high confidence of correct rejection (low probability of false rejection).<h4>Information needed for assessing compliance&nbsp;</h4>The following information is needed:&nbsp;<ul><li>measurand clearly specified;&nbsp;</li><li>specification stating upper or lower limit or both;&nbsp;</li><li>decision rule;&nbsp;</li><li>measured value:&nbsp;</li><li>measurement uncertainty for a measured value at the limit(s).</li></ul><h4>Example focusing on correct acceptance&nbsp;</h4><ul><li>Measurand: Mass fraction of nickel, Ni in a batch of steel delivered to a customer.&nbsp;</li><li>Uncertainty: U = 0.2 % Ni, k = 2 (95 %). Standard uncertainty, u = 0.1 % Ni. This uncertainty includes both sampling uncertainty for the batch and analytical uncertainty.&nbsp;</li><li>Specification: The specification zone is from the lower limit 16.0 % Ni to the upper limit 18.0 % Ni.&nbsp;</li><li>Decision rule High confidence of correct acceptance: The acceptance range is the mass fraction where it can be decided with a confidence level of approximately 95 % (α = 0.05) that the batch has a mass fraction above the lower limit and below the upper limit.&nbsp;</li><li>Guard band: Each guard band is calculated as 1.64u ≈ 0.17 % (rounded upward for safety) with k value 1.64 from the one-tailed upper 95 % quantile of the normal distribution.&nbsp;</li><li>Acceptance zone: 16.2 % Ni to 17.8 % Ni , after rounding to one decimal place.&nbsp;</li><li>Measured value: 16.1 % Ni</li></ul><img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Eurachem_Guard_bands_g_and_acceptance_and_a_rejection_zones_based_on_a_lower_and_upper_bedc4769b3.jpg" alt="Eurachem: Guard bands (g), and acceptance and a rejection zones based on a lower and upper limit and a decision rule stating high confidence of correct acceptance" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Eurachem_Guard_bands_g_and_acceptance_and_a_rejection_zones_based_on_a_lower_and_upper_bedc4769b3.jpg 245w," sizes="100vw" width="1091" height="447">The measured value, 16.1 % Ni, is below the lower acceptance limit of 16.2 %; that is, in the rejection zone. The batch is non-compliant.&nbsp;<blockquote>Note – if the decision rule stated simple acceptance the acceptance zone would be 16.0 % to 18.0 % and the batch would be compliant.&nbsp;</blockquote><h4>Example focusing on correct rejection&nbsp;</h4><ul><li>Measurand: Mass fraction of a banned substance in a sample.&nbsp;</li><li>Uncertainty: The relative standard uncertainty urel is 35 %.&nbsp;</li><li>Specification: Upper limit is 2 ng/g.&nbsp;</li><li>Decision rule High confidence of correct rejection: The concentration of the banned substance will be deemed to be above the limit if the probability of the value of the concentration being greater than the limit is 95 % or greater.</li><li>Guard band: The guard band g for correct rejection is 1.6 ng/g assuming a lognormal distribution (due to the high relative uncertainty – see further Guide Annex A, Case 4 [1]).&nbsp;</li><li>Acceptance limit: 3.6 ng/g&nbsp;</li><li>Measured value: 3.3 ng/g&nbsp;</li></ul><img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Eurachem_Guard_band_g_and_acceptance_and_a_rejection_zones_based_f0f8f140db.jpg" alt="Eurachem: Guard band (g), and acceptance and a rejection zones based on a decision rule stating high confidence of correct rejection" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Eurachem_Guard_band_g_and_acceptance_and_a_rejection_zones_based_f0f8f140db.jpg 245w," sizes="100vw" width="883" height="392">The measured value, 3.3 ng/g, is below the acceptance limit of 3.6 ng/g; that is, in the acceptance zone. The sample is compliant. Note that the assumption of the type of distribution is crucial. If assuming a normal distribution in this case the acceptance limit would be 3.2 ng/g and the sample would not be compliant.

Use of uncertainty in compliance

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Eurachem is a European network promoting international traceability of chemical measurements and good analytical practice. It provides expert guidance, supports method validation, uncertainty evaluation, proficiency testing, and facilitates discussion on technical and policy issues in analytical quality.

Use of uncertainty in compliance

Metrological Traceability of Analytical Results

Important information to our customers concerning the quality of measurements

Interlaboratory comparisons other than proficiency testing

Let’s call a PT scheme a PT scheme!

Understanding PT performance assessment

Understanding PT statistics

How to investigate poor performance in proficiency testing

Use of surplus proficiency test items

Proficiency testing schemes for sampling

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