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A Pragmatic Method for Pass/Fail Conformance Reporting that Complies with ANSI/NCSL Z540.3, ISO/IEC 17025, and ILAC-G8

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A Pragmatic Method for Pass/Fail Conformance Reporting that Complies with ANSI/NCSL Z540.3, ISO/IEC 17025, and ILAC-G8

Michael Dobbert and Robert Stern

Abstract: What are the criteria for stating Pass/Fail conformance when calibrating an instrument and comparing the measured results against specifications? The answer depends on regional and regulatory requirements, customer need and other criteria. This requires calibration service providers to be flexible when reporting calibration results which include Pass/Fail conformance statements. This is especially true when serving a global market. This paper explores the different requirements or guidelines in standards documents, such as ANSI/NCSL Z540.3-2006, ISO/IEC 17025:2005, ILAC-G8:1996, and EURAMET/cg-15/v.01. Some of these documents are prescriptive, while others provide only minimal guidance subject to interpretation. While many customers simply want to know pass or fail, these differences lead to variations in the Pass/Fail decision point, in the results labels (Pass/Fail vs. Pass/Indeterminate/Fail), and potentially have an effect on the downstream uncertainty analysis. This paper presents a non-obvious, yet simple method for expressing statements of Pass/Fail conformance. It employs flexible acceptance limits resulting in straightforward “Pass” and “Fail” conformance labels, with unobtrusive annotation to communicate additional information required by the standards documents. The result is a concise, uniform method flexible enough to satisfy all of the aforementioned standards, regardless of the chosen acceptance limits.

1. Introduction

The authors were part of a team of metrologists, engineers, and quality managers tasked with developing a new common measurement report for Agilent Technologies’ global calibration business. As an international measurement company, the challenge for Agilent Technologies was “how to satisfy multiple geographic region requirements in one standard report?” The team evaluated a series of Pass/Fail reporting designs before adopting the method reported in this paper. When making a statement of conformance, we must acknowledge the risk that the statement may be incorrect. Various calibration standards each address risk management in a different way. The key differences are:

• ANSI/NCSL Z540-1 [1]: Pass/Fail criteria was a simple comparison to the instrument manufacturer’s specified tolerance, so acceptance limits were equal to tolerance limits.

• ANSI/NCSL Z540.3 [2]: The probability of false acceptance (PFA) associated with any test point labeled “Pass” shall not exceed 2 %. (5.3 b)

• ISO/IEC 17025 [3]: States Pass/Fail criteria as, “When statements of compliance are made, the uncertainty of measurement shall be taken into account.” (5.10.4.2) Accreditation bodies provide local regional interpretation of the international standard.

• ILAC-G8:1996 [4]: Pass/Fail criteria uses the 95 % expanded uncertainty for making statements of conformance. For measured values where the specified tolerance is within the 95 % expanded uncertainty interval, no declaration of conformance is made. Most European accreditation bodies require ILAC-G8 for statements of conformance for ISO/IEC 17025 calibrations.

• EURAMET/cg-15/v.01 [5]: Though targeted for digital multimeters, EURAMET/cg-15/v.01 can be applied to other instruments. No guard band is applied when assessing conformance during calibration. “Subsequent to calibration and under normal conditions of use, the uncertainty associated with the readings of a DMM will be the combination of the DMM’s specification and the calibration uncertainty.” (4.2).

All the calibration standards above address risk management, with different approaches. However, each relies on the same fundamental risk concepts. The approach to managing risk in calibration plays a significant role in the application of acceptance limits and in the statement of conformance. Of particular concern is how to report measurement results that fall outside the acceptance limit, yet are within the manufacturer’s tolerance.

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