HOMOGENEITY of ring trial test items
The homogeneity of a proficiency testing (PT) item, also known as a ring trial test item, is crucial to ensure that the proficiency test results are reliable and comparable across different participants. ISO/IEC 17043:2023 specifies that an assessment of the homogeneity and stability of PT items, including ring trial test items, must be performed using valid statistical methods, based on the risk that differences among items could affect performance evaluation.
Various statistical approaches to sufficient homogeneity and stability assessment can include reviewing a representative sample of PT items randomly selected from the entire batch after final packaging, reviewing a small sample of PT items and comparing the differences among samples to criteria based on previous experiences with similar items, and reviewing participant data for consistency with previous rounds of the PT scheme.
Experimental evidence is often necessary for PT in new areas of application or when new procedures for producing PT items are used, especially for PT schemes using inherently inhomogeneous or unstable materials or consumer products. When a PT provider has experience with a specific type of PT item and production methods, there might be an insignificant risk of differences among items that could affect performance evaluation.
The assessment of PT item homogeneity may be like the evaluation of certified reference materials; however, it requires the estimation of uncertainties due to differences among items. Given the need for a reliable estimate for this component for certified properties, the number of randomly selected samples might exceed what is necessary for PT, where the primary goal is to verify differences that might impact performance evaluation.
In summary, homogeneity is a critical aspect in the organization and execution of PT schemes, ensuring that interlaboratory comparisons are fair and meaningful. Specific methods and considerations for homogeneity assessment must be carefully selected to reflect the objectives and context of each PT scheme.
STABILITY of ring trial test items
The stability of a proficiency testing (PT) item is a critical factor in ensuring the reliability and accuracy of PT exercises. According to ISO/IEC 17043:2023, assessing the stability of PT items involves determining that these items will remain unchanged and suitable for the intended assessment throughout the PT exercise, including during storage, handling, and transport. This ensures that any variability in the results can be attributed to the performance of the participants rather than to changes in the PT item itself.
To effectively assess the stability of PT items, ISO/IEC 17043:2023 outlines a process that includes establishing criteria based on the risk that instability could impact participant performance evaluation. The PT provider must document and conduct these assessments using appropriate statistical designs after the PT items have been prepared in their final form.
Stability can be demonstrated prior to packaging in cases where the packaging does not reasonably affect the item, or when stability studies indicate that the material should be stored in bulk form. Various approaches for stability assessment are described, including situations where experimental studies may not be feasible, as detailed in Annex B of ISO/IEC 17043:2023, ISO 13528, and ISO Guide 35.
When experimental evidence is required to assess the stability (or homogeneity) of the PT item, appropriate methods must be employed by the PT provider. This ensures the PT items are demonstrated to be sufficiently stable, preventing any significant change that might occur during the conduct of the PT round. If achieving stability is not possible, the instability must be quantified and considered as an additional component of the uncertainty associated with the PT item’s assigned value or factored into the evaluation criteria.
This comprehensive approach to assessing the stability of PT items underlines the importance of ensuring that PT exercises provide accurate and reliable assessments of participant performance, free from the influence of changes in the PT items themselves.
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