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ISO 13528:2015 Statistical methods for use in proficiency testing by interlaboratory comparison
ISO 13528:2015 Statistical methods for use in proficiency testing by interlaboratory comparison: Proficiency testing involves the use of interlaboratory comparisons to determine the performance of participants (which may be laboratories, inspection bodies, or individuals) for specific tests or measurements, and to monitor their continuing performance. There are a number of typical purposes of proficiency testing ISO/IEC 17043:2010. These include the evaluation of laboratory performance, the identification of problems in laboratories, establishing effectiveness and comparability of test or measurement methods, the provision of additional confidence to laboratory customers, validation of uncertainty claims, and the education of participating laboratories. The statistical design and analytical techniques applied must be appropriate for the stated purpose(s).
Abbreviation: ISO 13528:2015
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Publicly available informative sections of standards
- Rationale for scoring in proficiency testing schemes
- A variety of scoring strategies is available and in use for proficiency testing. Although the detailed calculations differ, most proficiency testing schemes compare the participant’s deviation from an assigned value with a numerical criterion which is used to decide whether or not the deviation represents cause for concern. The strategies used for value assignment and for choosing a criterion for assessment of the participant deviations are therefore critical. In particular, it is important to consider whether the assigned value and criterion for assessing deviations should be independent of participant results, or should be derived from the results submitted. In this Standard, both strategies are provided for. .. It will be seen that in general, choosing assigned values and assessment criteria independently of participant results offers advantages. This is particularly the case for the criterion used to assess deviations from the assigned value – such as the standard deviation for proficiency assessment or an allowance for measurement error – for which a consistent choice based on suitability for a particular end use of the measurement results, is especially useful.
- Statistical expertise
- ISO/IEC 17043:2010 requires that in order to be competent, a proficiency testing provider shall have access to statistical expertise and shall authorize specific personnel to conduct statistical analysis. Neither ISO/IEC 17043 nor this International Standard can specify further what that necessary expertise is. For some applications an advanced degree in statistics is useful, but usually the needs for expertise can be met by individuals with technical expertise in other areas, who are familiar with basic statistical concepts and have experience or training in the common techniques applicable to the analysis of data from proficiency testing schemes. If an individual is charged with statistical design and/or analysis, it is very important that this person has experience with interlaboratory comparisons, even if that person has an advanced degree in statistics. Conventional advanced statistical training often does not include exercises with interlaboratory comparisons, and the unique causes of measurement error that occur in proficiency testing can seem obscure. The guidance in this International Standard cannot provide all the necessary expertise to consider all applications, and cannot replace the experience gained by working with interlaboratory comparisons.
- Terms and definitions
- Interlaboratory comparison: organization, performance and evaluation of measurements or tests on the same or similar items by two or more laboratories in accordance with predetermined conditions
- Proficiency testing: evaluation of participant performance against pre-established criteria by means of interlaboratory comparisons.
- Assigned value: value attributed to a particular property of a proficiency test item.
- Standard deviation for proficiency assessment: measure of dispersion used in the evaluation of results of proficiency testing. This can be interpreted as the population standard deviation of results from a hypothetical population of laboratories performing exactly in accordance with requirements. The standard deviation for proficiency assessment applies only to ratio and interval scale results. Not all proficiency testing schemes evaluate performance based on the dispersion of results [SOURCE: ISO/IEC 17043:2010, modified — In the definition “, based on the available information” has been deleted. Note 1 to the entry has been added, and Notes 2 and 3 have been slightly edited].
- Measurement error: measured quantity value minus a reference quantity value [SOURCE: ISO/IEC Guide 99:2007, modified].
- Maximum permissible error: extreme value of measurement error, with respect to a known reference quantity value, permitted by specifications or regulations for a given measurement, measuring instrument, or measuring system [SOURCE: ISO/IEC Guide 99:2007, modified].
- z score: standardized measure of performance, calculated using the participant result, assigned value and the standard deviation for proficiency assessment. A common variation on the z score, sometimes denoted z’ (commonly pronounced z-prime), is formed by combining the uncertainty of the assigned value with the standard deviation for proficiency assessment before calculating the z score.
- zeta score: standardized measure of performance, calculated using the participant result, assigned value and the combined standard uncertainties for the result and the assigned value.
- Proportion of allowed limit score: standardized measure of performance, calculated using the participant result, assigned value and the criterion for measurement error in a proficiency test. For single results, performance can be expressed as the deviation from the assigned value (D or D %).
- Action signal: indication of a need for action arising from a proficiency test result. EXAMPLE: A z score in excess of 2 is conventionally taken as an indication of a need to investigate possible causes; a z score in excess of 3 is conventionally taken as an action signal indicating a need for corrective action.
- Consensus value: value derived from a collection of results in an interlaboratory comparison. The phrase ‘consensus value’ is typically used to describe estimates of location and dispersion derived from participant results in a proficiency test round, but may also be used to refer to values derived from results of a specified subset of such results or, for example, from a number of expert laboratories.
- Outlier: member of a set of values which is inconsistent with other members of that set. An outlier can arise by chance from the expected population, originate from a different population, or be the result of an incorrect recording or other blunder. Many schemes use the term outlier to designate a result that generates an action signal. This is not the intended use of the term. While outliers will usually generate action signals, it is possible to have action signals from results that are not outliers [SOURCE: ISO 5725‑1:1994, modified].
- Participant: laboratory, organization, or individual that receives proficiency test items and submits results for review by the proficiency testing provider.
- Proficiency test item: sample, product, artefact, reference material, piece of equipment, measurement standard, data set or other information used to assess participant performance in proficiency testing. In most instances, proficiency test items meet the ISO Guide 30 definition of “reference material”.
- Proficiency testing provider: organization which takes responsibility for all tasks in the development and operation of a proficiency testing scheme.
- Proficiency testing scheme: proficiency testing designed and operated in one or more rounds for a specified area of testing, measurement, calibration or inspection. A proficiency testing scheme might cover a particular type of test, calibration, inspection or a number of tests, calibrations or inspections on proficiency test items.
- Reference material (RM): material, sufficiently homogeneous and stable with respect to one or more specified properties, which has been established to be fit for its intended use in a measurement process. RM is a generic term. Properties can be quantitative or qualitative, e.g. identity of substances or species. Uses may include the calibration of a measuring system, assessment of a measurement procedure, assigning values to other materials, and quality control [SOURCE: ISO Guide 30:2015, modified].
- Certified reference material (CRM): reference material (RM) characterized by a metrologically valid procedure for one or more specified properties, accompanied by an RM certificate that provides the value of the specified property, its associated uncertainty, and a statement of metrological traceability. The concept of value includes a nominal property or a qualitative attributes such as identity or sequence. Uncertainties for such attributes may be expressed as probabilities or levels of confidence.