{ "id": "epr97typ88", "slug": "ring-test", "title": "Ring test", "updated": "2026-10-15", "topic_path": [ { "id": "physical", "label": "Physical world and mathematics", "api_url": "https://www.edgepedia.org/api/v1/topics/physical" }, { "id": "physical.measurement", "label": "Measurement and time", "api_url": "https://www.edgepedia.org/api/v1/topics/physical.measurement" }, { "id": "physical.measurement.metrology-and-instrumentation", "label": "Metrology, instrumentation, and applied measurement", "api_url": "https://www.edgepedia.org/api/v1/topics/physical.measurement.metrology-and-instrumentation" }, { "id": "physical.measurement.metrology-overview", "label": "Metrology and measurement science (overview)", "api_url": "https://www.edgepedia.org/api/v1/topics/physical.measurement.metrology-overview" } ], "geo": [ { "id": "geo.nongeo.t1946.physical", "label": "Non-geographic · 1946 to 2000: Physical world and mathematics", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.physical", "path": [ { "id": "geo.nongeo", "label": "Non-geographic", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo" }, { "id": "geo.nongeo.t1946", "label": "Non-geographic · 1946 to 2000", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946" }, { "id": "geo.nongeo.t1946.physical", "label": "Physical world and mathematics", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.physical" } ] } ], "excerpt": "A ring test, also called a ring trial or round robin, is an interlaboratory comparison in which identical samples are analyzed and scored against an assigned value.", "snippet": "A ring test, also called a ring trial or round robin, is an interlaboratory comparison in which identical samples are analyzed and scored against an assigned value.", "node": "physical.measurement.metrology-overview", "markdown": "# Ring test\n\nA ring test is an interlaboratory comparison in which a reference institute or scheme provider sends identical samples to a number of laboratories, each of which must analyze them for specified parameters and report results by a deadline.[1](https://goldbook.iupac.org/terms/view/14996) The provider compares every participant's result against an assigned value and converts each deviation into a standardized score that tells the laboratory how its accuracy compares with pre-established criteria.[2](https://pt-drw.aafco.org/CSPStatsDocs/Reports/AMC_TB2.pdf) • [3](https://cdn-v2.iasonline.org/wp-content/uploads/2024/12/IAS_ADM_103-IAS-Policy-on-Proficiency-Testing-20241216.pdf) The exercise is one form of proficiency testing (PT), also called external quality assessment (EQA), round robin, or ring trial.[4](https://lab.fnri.dost.gov.ph/ptl/frontend/web/manual/PT-Protocol-7th-Ed.pdf) Providers operate under ISO/IEC 17043, and the statistics follow ISO 13528.[5](https://www.iso.org/standard/80864.html) • [6](https://www.iso.org/standard/78879.html)\n\n| Key fact | Detail |\n|---|---|\n| Definition | Identical samples sent to many laboratories for analysis of specified parameters by a deadline[1](https://goldbook.iupac.org/terms/view/14996) |\n| Governing standards | ISO/IEC 17043:2023 (providers); ISO 13528:2022 (statistics)[5](https://www.iso.org/standard/80864.html) • [6](https://www.iso.org/standard/78879.html) |\n| Main scores | z, z′, zeta (ζ), En, percent difference D%[7](https://webstore.ansi.org/preview-pages/DS/preview_M362852CURES.pdf) |\n| Conventional thresholds | \\|z\\| ≤ 2.0 satisfactory; 2.0 < \\|z\\| < 3.0 warning; \\|z\\| ≥ 3.0 action signal[8](https://eurachem.org/images/stories/leaflets/pt/pt_perf_assmnt/PT_Understand_Evaluation_V1_EN.pdf) |\n| Assigned-value uncertainty rule | Use z′ when \\( u(x_{\\mathrm{pt}}) > 0.3 \\cdot \\sigma_{\\mathrm{pt}} \\)[8](https://eurachem.org/images/stories/leaflets/pt/pt_perf_assmnt/PT_Understand_Evaluation_V1_EN.pdf) |\n| Homogeneity criterion | \\( s_{\\mathrm{sam}} \\le 0.3 \\cdot \\sigma \\), tested on 10 samples in duplicate[9](https://lab.fnri.dost.gov.ph/ptl/frontend/web/manual/PT-Statistical-Protocol-8th.pdf) |\n| Collusion control | Assigned value withheld until the round closes; late results refused[4](https://lab.fnri.dost.gov.ph/ptl/frontend/web/manual/PT-Protocol-7th-Ed.pdf) |\n\n## How it works\n\nEvery performance assessment is based on the difference between the participant's result \\( x_{i} \\) and the assigned value \\( x_{pt} \\), divided by a normalization factor.[8](https://eurachem.org/images/stories/leaflets/pt/pt_perf_assmnt/PT_Understand_Evaluation_V1_EN.pdf) The underlying statistical model treats each result as \\( x_{i} = m + B_{i} + e \\), a sum of the consensus mean, a laboratory-specific bias, and random error.[10](https://www.itc.gov.hk/en/quality/hkas/doc/common/presentation/ISO_13528_review.pdf)\n\nThe **z-score** divides the deviation by the standard deviation for proficiency assessment (\\( \\sigma_{\\mathrm{pt}} \\)):\n\n\\[ z = \\frac{x - x_{pt}}{\\sigma_{pt}} \\]\n\nUnder a Gaussian assumption, about 95% of z-scores fall between −2 and +2, and a score outside ±3 is taken to indicate a cause that should be investigated and remedied.[11](https://list.iupac.org/publications/pac/2006/pdf/7801x0145.pdf) ISO 13528:2022 tabulates the interpretation as |score| ≤ 2.0 satisfactory, 2.0 < |score| < 3.0 a warning signal, and |score| ≥ 3.0 an action signal indicating unsatisfactory performance, for z, z′, and zeta scores alike.[8](https://eurachem.org/images/stories/leaflets/pt/pt_perf_assmnt/PT_Understand_Evaluation_V1_EN.pdf)\n\nWhen the uncertainty of the assigned value is significant, \\( u(x_{\\mathrm{pt}}) > 0.3 \\cdot \\sigma_{\\mathrm{pt}} \\), the **z′ score** is used instead, with the denominator \\( \\sqrt{\\sigma_{\\mathrm{pt}}^{2}+u^{2}(x_{\\mathrm{pt}})} \\), formed by combining the uncertainty of the assigned value with the standard deviation for proficiency assessment.[8](https://eurachem.org/images/stories/leaflets/pt/pt_perf_assmnt/PT_Understand_Evaluation_V1_EN.pdf) • [12](https://cdn.standards.iteh.ai/samples/78879/63a7d287acda4f4fad482b038a6b75d3/ISO-13528-2022.pdf) The **zeta (ζ) score** standardizes the deviation using the combined standard uncertainties of both the result and the assigned value.[12](https://cdn.standards.iteh.ai/samples/78879/63a7d287acda4f4fad482b038a6b75d3/ISO-13528-2022.pdf) The **En score** uses a combination of expanded uncertainties corresponding to roughly a 95% confidence interval and is common in metrology comparisons among calibration laboratories; |En| ≤ 1.0 can indicate satisfactory performance and |En| > 1.0 a need to review the evaluated uncertainty or the measurement procedure.[8](https://eurachem.org/images/stories/leaflets/pt/pt_perf_assmnt/PT_Understand_Evaluation_V1_EN.pdf) • [13](https://www.eurachem.org/images/stories/Guides/pdf/EPT_2021_P1_EN.pdf) The **percent difference D%** expresses the deviation as a percentage of \\( x_{\\mathrm{pt}} \\) against a maximum permissible relative deviation \\( d_{E\\%} = 100 \\cdot d_{E} / |x_{\\mathrm{pt}}| \\), expressed as a percentage, set by the provider.[8](https://eurachem.org/images/stories/leaflets/pt/pt_perf_assmnt/PT_Understand_Evaluation_V1_EN.pdf)\n\n## How it is done\n\nA coordinator organizes preparation, homogeneity testing and validation of the test material, distributes samples on a regular schedule, receives centrally reported results, performs the statistical analysis, and assesses laboratory performance, with advice available to poor performers.[14](https://rsync.iupac.org/publications/pac/1993/pdf/6509x2123.pdf) A typical round runs through preparation of the test item, homogeneity and stability tests, registration, dispatch, participant analysis (typically one month from dispatch), collation, statistical evaluation, interim and final reports, and a post-round meeting.[4](https://lab.fnri.dost.gov.ph/ptl/frontend/web/manual/PT-Protocol-7th-Ed.pdf)\n\nTest material is prepared by spiking the relevant analytes or by using material with incurred analytes, and the provider must demonstrate homogeneity and stability before distribution.[15](https://www.lach-bruns.de/wp-content/uploads/2019/07/LuB-guideline-for-ring-tests-20190701.pdf) Items are not distributed until testing shows sufficient homogeneity under the ISO 13528:2022 procedure.[4](https://lab.fnri.dost.gov.ph/ptl/frontend/web/manual/PT-Protocol-7th-Ed.pdf) Participants report one value per measurand in a prescribed format, plus an expanded measurement uncertainty at a prescribed confidence level and coverage factor.[4](https://lab.fnri.dost.gov.ph/ptl/frontend/web/manual/PT-Protocol-7th-Ed.pdf) Final reports state \\( \\sigma_{\\mathrm{pt}} \\), the assigned values \\( x_{\\mathrm{pt}} \\), their standard uncertainties \\( u(x_{\\mathrm{pt}}) \\), and the computed z or z′ scores, alongside summary statistics and plots of ordered results.[4](https://lab.fnri.dost.gov.ph/ptl/frontend/web/manual/PT-Protocol-7th-Ed.pdf)\n\n## Origin\n\n Documented milestones begin with an Office of Weights and Measures PT program, conducted as \"round robin\" tests supporting metrological traceability of state laws to national and international standards.[16](https://www.nist.gov/publications/nistir-7082-proficiency-testing-policy-and-plan-state-weights-and-measures-laboratories) The International Harmonized Protocol for the Proficiency Testing of (Chemical) Analytical Laboratories codified the ring-test cycle and the z-score framework.[14](https://rsync.iupac.org/publications/pac/1993/pdf/6509x2123.pdf) • [11](https://list.iupac.org/publications/pac/2006/pdf/7801x0145.pdf) ISO 13528:2022 incorporates the published IUPAC guidance for chemical laboratories while extending the procedures to a wider range of measurement methods and qualitative identifications.[7](https://webstore.ansi.org/preview-pages/DS/preview_M362852CURES.pdf)\n\n## Variants\n\nISO/IEC 17043 distinguishes a sequential scheme, in which a unique item is transferred between participants, from a simultaneous scheme, in which similar items from a batch are distributed at the same time.[17](https://icpms.labrulez.com/article/7509) ISO 13528 likewise recognizes concurrent testing within a defined time period, single-occasion exercises, and continuous schemes with items provided at regular intervals.[18](https://cdn.standards.iteh.ai/samples/56125/afcddf885d3746e6a839fc5147c7d945/ISO-13528-2015.pdf) In metrology, artifact circulation can be round-shaped, with the item passed successively from laboratory to laboratory, or radial (star-shaped), with the reference value determined at the beginning, end, or both of a round-shaped run.[19](https://www.ptb.de/cms/fileadmin/internet/fachabteilungen/abteilung_q/q.3_internationale_zusammenarbeit/publikationen/Studies/PTB_Study_Guide_Proficiency_Testing_EN.pdf)\n\nInterlaboratory comparisons evaluating a candidate standard method are called method performance or collaborative studies; those producing certified reference materials are material certification studies; and those demonstrating National Metrology Institutes' capabilities are key comparisons, supplementary comparisons, or pilot studies.[17](https://icpms.labrulez.com/article/7509) • [19](https://www.ptb.de/cms/fileadmin/internet/fachabteilungen/abteilung_q/q.3_internationale_zusammenarbeit/publikationen/Studies/PTB_Study_Guide_Proficiency_Testing_EN.pdf)\n\n## Applications\n\nRing tests and PT schemes operate across chemical and food analysis, calibration, and medical laboratories. Food and feed examples include the Gafta ring test scheme[20](https://www.gafta.com/media/tnml0y1s/gafta-ring-test-scheme-protocol-v2-aug-2026.pdf) and the FNRI protocol for nutrition laboratories.[4](https://lab.fnri.dost.gov.ph/ptl/frontend/web/manual/PT-Protocol-7th-Ed.pdf) In calibration, En scores are the usual currency of comparisons among calibration laboratories.[8](https://eurachem.org/images/stories/leaflets/pt/pt_perf_assmnt/PT_Understand_Evaluation_V1_EN.pdf) PT plays a key role for laboratories seeking or maintaining [ISO/IEC 17025](https://www.edgepedia.org/iso-iec-17025) accreditation, complementing on-site assessment.[19](https://www.ptb.de/cms/fileadmin/internet/fachabteilungen/abteilung_q/q.3_internationale_zusammenarbeit/publikationen/Studies/PTB_Study_Guide_Proficiency_Testing_EN.pdf) • [21](https://pta.asn.au/wp-content/uploads/2025/04/Guide-to-Proficiency-Testing-Australia.pdf) Medical laboratories accredited under [ISO 15189](https://www.edgepedia.org/iso-15189) must participate in appropriate interlaboratory comparisons, including PT when suitable and available, and demonstrate the validity of results by other means when PT is unavailable or inappropriate.[22](https://doi.org/10.20506/rst.40.1.3217)\n\n## Limitations and alternatives\n\n**Sample quality is the first failure mode.** ISO 13528 requires homogeneity and stability criteria based on the effect that inhomogeneity and instability will have on the evaluation of participants' performance.[10](https://www.itc.gov.hk/en/quality/hkas/doc/common/presentation/ISO_13528_review.pdf) Homogeneity testing uses 10 test samples in duplicate under repeatability conditions, with the allowable sampling variance \\( \\sigma^{2}_{\\mathrm{allow}} = (0.3 \\cdot \\sigma_{\\mathrm{pt}})^{2} \\); samples are adequately homogeneous if \\( s_{\\mathrm{sam}} \\le 0.3 \\cdot \\sigma \\).[9](https://lab.fnri.dost.gov.ph/ptl/frontend/web/manual/PT-Statistical-Protocol-8th.pdf) • [23](https://www.industry.gov.au/sites/default/files/2019-05/cptstatisticalmanual.pdf) A stability check compares averages before and after the round: the item is stable if |ȳ₁ − ȳ₂| < 0.3·σ_pt, and if instability is confirmed the provider issues z′ scores instead.[9](https://lab.fnri.dost.gov.ph/ptl/frontend/web/manual/PT-Statistical-Protocol-8th.pdf) Assignment of target values from weighed-in material depends on the purity of the material, the accuracy of the measurement devices, and the demonstrated similarity between the pure material and the test form.[24](https://www.skml.nl/app/uploads/2023/12/Proficiency-Testing-External-Quality-Assessment.pdf)\n\n**Collusion** is a recognized failure mode; rounds prevent it by keeping assigned values secret until the round closes and refusing results submitted after the assigned value has been issued.[4](https://lab.fnri.dost.gov.ph/ptl/frontend/web/manual/PT-Protocol-7th-Ed.pdf)\n\n**Score interpretation has caveats.** The z′ score standardizes the deviation but fails to differentiate between a poor result and a poor assigned value; a large zeta score can indicate a large error, an underestimated uncertainty, or both.[13](https://www.eurachem.org/images/stories/Guides/pdf/EPT_2021_P1_EN.pdf) Published guidance also differs on when the assigned value's uncertainty invalidates plain z-scores: the IUPAC protocol recommends not presenting unqualified z-scores when \\( u^{2}(x_{\\mathrm{pt}}) > 0.1 \\cdot \\sigma^{2}_{\\mathrm{pt}} \\),[11](https://list.iupac.org/publications/pac/2006/pdf/7801x0145.pdf) while ISO 13528:2022 and Eurachem use the threshold \\( u(x_{\\mathrm{pt}}) > 0.3 \\cdot \\sigma_{\\mathrm{pt}} \\) (equivalent to \\( u^{2} > 0.09 \\cdot \\sigma^{2}_{\\mathrm{pt}} \\)).[8](https://eurachem.org/images/stories/leaflets/pt/pt_perf_assmnt/PT_Understand_Evaluation_V1_EN.pdf)\n\n**Failing a round triggers documented investigation.** Every unsatisfactory performance score should be investigated and documented, with depth depending on the criticality of the measurement, the frequency of unsatisfactory results, and evidence of bias.[13](https://www.eurachem.org/images/stories/Guides/pdf/EPT_2021_P1_EN.pdf) Laboratories with |z or z′| > 2.00 are encouraged to review, investigate, and take corrective action to prevent recurrence.[4](https://lab.fnri.dost.gov.ph/ptl/frontend/web/manual/PT-Protocol-7th-Ed.pdf) [Accreditation](https://www.edgepedia.org/accreditation) bodies raise a non-conformity when a laboratory with non-satisfactory performance has not implemented prompt and appropriate corrective actions.[3](https://cdn-v2.iasonline.org/wp-content/uploads/2024/12/IAS_ADM_103-IAS-Policy-on-Proficiency-Testing-20241216.pdf) As an alternative to multi-laboratory rounds, a measurement audit compares a single laboratory's result on a calibrated item against a reference value.[21](https://pta.asn.au/wp-content/uploads/2025/04/Guide-to-Proficiency-Testing-Australia.pdf)\n\n## References\n\n1. [IUPAC Gold Book: ring test](https://goldbook.iupac.org/terms/view/14996)\n2. [AMC Technical Brief (No. 2)](https://pt-drw.aafco.org/CSPStatsDocs/Reports/AMC_TB2.pdf)\n3. [IAS Policy on Participation in Proficiency Testing and/or Interlaboratory Comparisons (2024-12-16)](https://cdn-v2.iasonline.org/wp-content/uploads/2024/12/IAS_ADM_103-IAS-Policy-on-Proficiency-Testing-20241216.pdf)\n4. [FNRI PT Protocol, 7th edition](https://lab.fnri.dost.gov.ph/ptl/frontend/web/manual/PT-Protocol-7th-Ed.pdf)\n5. [ISO/IEC 17043:2023, Conformity assessment, General requirements for the competence of proficiency testing providers](https://www.iso.org/standard/80864.html)\n6. [ISO 13528:2022 - Statistical methods for use in proficiency testing by interlaboratory comparison](https://www.iso.org/standard/78879.html)\n7. [ISO 13528:2022 preview (ANSI webstore)](https://webstore.ansi.org/preview-pages/DS/preview_M362852CURES.pdf)\n8. [Understanding PT performance assessment (Eurachem leaflet, May 2024)](https://eurachem.org/images/stories/leaflets/pt/pt_perf_assmnt/PT_Understand_Evaluation_V1_EN.pdf)\n9. [FNRI Statistical Protocol, 8th edition](https://lab.fnri.dost.gov.ph/ptl/frontend/web/manual/PT-Statistical-Protocol-8th.pdf)\n10. [ISO TC69 Update – Application of Statistical Methods (ISO 13528 review)](https://www.itc.gov.hk/en/quality/hkas/doc/common/presentation/ISO_13528_review.pdf)\n11. [The International Harmonized Protocol for the Proficiency Testing of Analytical Chemistry Laboratories (IUPAC, 2006)](https://list.iupac.org/publications/pac/2006/pdf/7801x0145.pdf)\n12. [ISO 13528:2022, Statistical methods for use in proficiency testing by interlaboratory comparison (preview)](https://cdn.standards.iteh.ai/samples/78879/63a7d287acda4f4fad482b038a6b75d3/ISO-13528-2022.pdf)\n13. [Selection, Use and Interpretation of Proficiency Testing (PT) Schemes (Eurachem Guide, 2021)](https://www.eurachem.org/images/stories/Guides/pdf/EPT_2021_P1_EN.pdf)\n14. [The International Harmonized Protocol for the Proficiency Testing of (Chemical) Analytical Laboratories](https://rsync.iupac.org/publications/pac/1993/pdf/6509x2123.pdf)\n15. [LuB guideline for ring tests (2019)](https://www.lach-bruns.de/wp-content/uploads/2019/07/LuB-guideline-for-ring-tests-20190701.pdf)\n16. [NISTIR 7082: Proficiency Testing Policy and Plan for State Weights and Measures Laboratories (2026 Ed.)](https://www.nist.gov/publications/nistir-7082-proficiency-testing-policy-and-plan-state-weights-and-measures-laboratories)\n17. [Let's call a PT scheme a PT scheme! | LabRulez ICPMS](https://icpms.labrulez.com/article/7509)\n18. [ISO 13528:2015 (sample preview)](https://cdn.standards.iteh.ai/samples/56125/afcddf885d3746e6a839fc5147c7d945/ISO-13528-2015.pdf)\n19. [PTB Study Guide: Proficiency Testing](https://www.ptb.de/cms/fileadmin/internet/fachabteilungen/abteilung_q/q.3_internationale_zusammenarbeit/publikationen/Studies/PTB_Study_Guide_Proficiency_Testing_EN.pdf)\n20. [Gafta Ring Test Scheme – Rules & Protocol (v2, Aug 2026)](https://www.gafta.com/media/tnml0y1s/gafta-ring-test-scheme-protocol-v2-aug-2026.pdf)\n21. [Guide to Proficiency Testing Australia](https://pta.asn.au/wp-content/uploads/2025/04/Guide-to-Proficiency-Testing-Australia.pdf)\n22. [Proficiency testing and ring trials](https://doi.org/10.20506/rst.40.1.3217)\n23. [CPT Statistical Manual v3.9 (NATA/National Measurement Institute, Australia)](https://www.industry.gov.au/sites/default/files/2019-05/cptstatisticalmanual.pdf)\n24. [Proficiency Testing/External Quality Assessment: Current Challenges and Future Directions](https://www.skml.nl/app/uploads/2023/12/Proficiency-Testing-External-Quality-Assessment.pdf)\n\n---\n*Topic: Encyclopedia › Physical world and mathematics › Measurement and time › Metrology, instrumentation, and applied measurement › Metrology and measurement science (overview)*\n\n*Initially written Sep 29, 2026 · Reviewed: Sep 30, 2026 · Edited: Oct 15, 2026 · Last review: Sep 30, 2026*\n\n*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*\n\nLicense: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license\n", "same_as": [], "url": "https://www.edgepedia.org/ring-test", "markdown_url": "https://www.edgepedia.org/ring-test.md", "license": { "name": "Edgepedia Community License 1.0", "url": "https://www.edgepedia.org/edgepedia/license", "summary": "Free with credit, commercial use included. AI training is open to everyone. For other uses, organizations over USD 100M in revenue or 100M monthly users license separately.", "spdx": "LicenseRef-Edgepedia-Community-1.0" }, "credit": "\"Ring test\", Edgepedia (EdgeChat), https://www.edgepedia.org/ring-test. Edgepedia Community License 1.0.", "credit_md": "\"[Ring test](https://www.edgepedia.org/ring-test)\", Edgepedia (EdgeChat), [https://www.edgepedia.org/ring-test](https://www.edgepedia.org/ring-test). [Edgepedia Community License 1.0](https://www.edgepedia.org/edgepedia/license).", "credit_html": "\"Ring test\", Edgepedia (EdgeChat), https://www.edgepedia.org/ring-test. Edgepedia Community License 1.0.", "speakable": "A ring test, also called a ring trial or round robin, is an interlaboratory comparison in which identical samples are analyzed and scored against an assigned value." }