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Guide to proficiency test results interpretation

05 May 2016  | Chris Grachanen

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When you bring a measurement instrument to a calibration lab, there is an assumption that your instrument will come back within expected tolerances. But, how do you know? That's where laboratory proficiency tests play a role.

For many calibration and testing laboratories, routine proficiency testing is a requirement for obtaining and maintaining accreditation status. ISO/IEC 17043, "Conformity assessment—General requirements for proficiency testing," defines proficiency testing as an "evaluation of participant performance against pre-established criteria by means of interlaboratory comparisons." Proficiency testing is analogous to a surveillance activity for assessing the quality and uniformity of tests and measurements performed in a laboratory.

ISO 17025 Section 5.9, "Assuring the Quality of Test and Calibration Results," references proficiency testing programs. Section 5.9.1 states:

The laboratory shall have quality control procedures for monitoring the validity of tests and calibrations undertaken. The resulting data shall be applied to the reviewing of the results. This monitoring shall be planned and reviewed and may include, but not be limited to, the following: use of CRMs and/or RMs; proficiency-testing; replicate tests; retesting; and correlation of results for different characteristics.

Looking up the definition of "assure," you'll see words such as promise, guarantee, pledge, convince, declare, give surety, and comfort. Given these definitions, proficiency testing is basically an assessment activity used to demonstrate or determine a level of work competence as derived from measuring a test artifact and evaluating the results. The demonstrated work-competence level is assumed to be representative of the relevant work performed by a laboratory. As such, it is deemed a predictor of sorts in regards to the work a laboratory may be expected to perform. A laboratory that satisfactorily completes a proficiency test provides an assumption that the laboratory will continue to make satisfactory measurements within the scope of the test.

Keep in mind that ILCs (Interlaboratory Comparison Programs) and MAPs (Measurement Assurance Programs don't necessarily demonstrate proficiency and are usually not accepted as meeting the proficiency requirements for accreditation unless specifically designed to meet these requirements. You can reference ISO/IEC 17043 for a better understanding of proficiency test requirements.

The methodology for performance determination is established through measurements made on test artifacts. Normally, test artifacts devices have established performance attributes that are relatively stable over time and exhibit characteristics similar to units tested or calibrated by a laboratory.

An example of a test artifact often used in pressure proficiency testing is a pressure transducer. Other proficiency tests often use test materials, commonly referred to as reference material or SRM (standard reference material) or CRM (certified reference material) is the norm. This test material is sufficiently stable and homogeneous regarding one or more of its properties, means it can be used in the assessment of measurement processes.

The selection of a test artifact or test material for a particular measurement parameter takes into consideration a laboratory's measurement capability in terms of measurement range, measurement accuracy, physical measurement restraints, safety considerations, etc. Characteristic values for proficiency test artifacts or test materials are normally assigned by a reputable, independent laboratory—a laboratory normally not participating in the proficiency test—which avoids the appearance of any bias towards a particular laboratory participating in a proficiency test.

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