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Measurement Accuracy Explained

 
https://gallery.mailchimp.com/1a69c7aaf35adde9f75b36e90/images/9fa53cbe-64cd-493c-b95a-c4665eee5f26.jpgInconclusive, Near Limits, Not Applicable, Borderline… These are just some of the terms used to describe test results approaching the pass/fail limits defined by test standards. But why are they used? This is because of inherent measurement accuracy/uncertainty, a range that is calculated per test type. This is an essential test attribute that all credible test reports must include. If no measurement accuracy/uncertainty is stated in your test report, proceed with caution, you may not pass (or fail) the test after all.

How is this derived? The range is derived from the known equipment accuracy, environment accuracy, test repeatability, and other variables which may affect the outcome of the result. Once the measurement accuracy of the test is determined, any test results that are within the measurement accuracy range from the defined pass/fail number set for the test, cannot be considered a pass or a fail. For example, if the measurement accuracy is plus or minus 0.1, and the minimum requirement specified by the standard is 5, this means that any result between 4.9 and 5.1 cannot be concluded as a fail or pass of the test (non-compliant or compliant). Instead, the conclusion must be one of the aforementioned terms. The measurement accuracy is required to be stated by ISO/IEC 17025 accredited labs for all results that could be affected by accuracy.

Why is this important?

Example 1) When test lab A produces a result of 58.5, and test B produces a result of 44.3, but both labs have a measurement uncertainty of plus or minus 10, we learn that the test has a low measurement accuracy. In this case, the inherent repeatability of this test is low, but the results from both labs are valid, even though the results seem quite different.

Example 2) A card is tested for the height dimension and reported as 52.922 mm, with the ISO/IEC 7810 standard specifying a minimum requirement of 52.920 mm. This test result may seem to be within the ISO limit. However, the tests measurement accuracy is plus or minus 0.005 mm, which means the actual real size of the card could be as low as 52.917 mm which fails the ISO requirement, or as high as 52.927 which passes the ISO requirement.



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