ISO 13528 Statistical Methods for Use in Proficiency Testing by Interlaboratory Comparison

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ISO 13528 Statistical Methods for Use in Proficiency Testing by Interlaboratory Comparison

EUROLAB laboratory provides testing and compliance services within the scope of ISO 13528 standard. The ISO 13528 standard, developed by the International Organization for Standardization (ISO), provides detailed descriptions of statistical methods for proficiency testing providers to use to design proficiency testing schemes and analyze data from these programmes. This Standard provides recommendations on the interpretation of proficiency testing data by accreditation bodies and participants in such programs.

ISO 13528 Statistical Methods for Use in Proficiency Testing by Interlaboratory Comparison

The procedures in this Standard can be applied to demonstrate that measurement results obtained by laboratories, inspection bodies and individuals meet specified criteria for acceptable performance. This Standard is applicable to proficiency tests where reported results are quantitative measurements or qualitative observations on test items.

The procedures in this Standard may also be applied to the assessment of expert opinion where opinions or judgments are reported in a form that is objectively comparable to an independent reference value or a consensus statistic. For example, when classifying proficiency test items into known categories by inspection - or in determining whether inspection and proficiency test items originate from the same original source - and objectively comparing classification results, the following provisions of this Standard may relate to nominal (qualitative) properties.

Purposes of Proficiency Testing

Proficiency testing involves the use of interlaboratory comparisons to identify and monitor the ongoing performance of participants (whether laboratories, inspection bodies or individuals) for specific tests or measures. There are several typical purposes of proficiency testing, as explained in the introduction to ISO/IEC 17043. These include evaluating laboratory performance, identifying problems in laboratories, determining the effectiveness and comparability of testing or measurement methods, providing additional confidence to laboratory customers, validating uncertainty claims, and training participating laboratories. Statistical design and analytical techniques applied should be suitable for the stated purposes.

Rationale for Scoring in Proficiency Test Schemes

Various scoring strategies exist and are used for proficiency testing. Although detailed calculations vary, most proficiency testing schemes compare a participant's deviation from an assigned value to a numerical criterion used to decide whether the deviation is a cause for concern. The strategies used to assign value and select a criterion for assessing participant bias are therefore critical. In particular, it is important to consider whether the value and criteria assigned to evaluate deviations should be independent of participant outcomes or derived from the outcomes presented. This Standard provides both strategies.

Overall, it will appear that the selection of assigned values ​​and evaluation criteria independently of participant outcomes offers advantages. This is particularly useful for the criterion used to evaluate deviations from the assigned value – such as the standard deviation for adequacy assessment or an allowance for measurement error – where a consistent choice is made based on suitability for a particular end use of the measurement results.

ISO 13528 provides support for the implementation of ISO/IEC 17043, particularly on the requirements for statistical design, verification of proficiency testing elements, review of results, and reporting of summary statistics. ISO/IEC 17043:2010 Annex B briefly describes the general statistical methods used in proficiency testing schemes. This International Standard is intended to complement ISO/IEC 17043 and provide detailed guidance on specific statistical methods for proficiency testing that is missing from this document.

The definition of proficiency testing in ISO/IEC 17043 is repeated in ISO 13528 with Notes explaining the different types of proficiency testing and the types of designs that can be used. This Standard cannot specifically cover all purposes, designs, matrices and measurands. The techniques presented in ISO 13528 are intended to be broadly applicable, particularly for newly established proficiency testing schemes.

The statistical techniques used for a particular proficiency testing plan are expected to improve as the plan matures; and scores, evaluation criteria and graphic techniques will be developed to better serve the specific needs of a target audience, accreditation bodies and regulatory authorities.

The computer software required for statistical analysis of proficiency test data can vary widely, from simple spreadsheet arithmetic for small proficiency test schemes using known reference values ​​to complex statistical software used for statistical methods based on iterative calculations or other advanced numerical methods.

Many of the techniques in this International Standard can be performed with traditional spreadsheet applications, perhaps with routines customized for a particular schema or analysis; some techniques will require computer applications that are freely available (at the time of publication of this Standard). In all cases, users should verify the accuracy of their calculations, especially when custom routines are entered by the user.

Among the services provided by our organization within the framework of material testing services, there are also ISO 13528 standard tests. Do not hesitate to contact our laboratory EUROLAB for your testing and certification requests.

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