Reporting Measurement Uncertainty

Three beakers that can yield different measurements, showing the importance of labs reporting measurement uncertainty.

Many are familiar with the saying, “Measure twice, cut once,” which stresses the importance of being diligent when performing a measurement to ensure its accuracy. The accuracy of any measurement is dependent on the measurement process and can be qualified by and expressed as the measurement uncertainty. 

It is the responsibility of any accredited laboratory performing measurements to provide accurate measurement results to their customers. Organizations and customers make decisions based on the information, data, and the measurement results provided. They rely on these measurement results to be quantified and reported accurately and the reported results to be clear, unambiguous, and objective. 

When a measurement is not quantified with accuracy, or the result not reported with sufficient and appropriate qualification, there is an increased risk of misinterpretation of that measurement result. A decision made based on a misinterpretation could subsequently lead to a detrimental action that could put the organization’s reputation and resources at risk.

Measurements and Estimating Measurement Uncertainty

How does the laboratory ensure accuracy of measurements and what is involved with estimating measurement uncertainty? First, we need to understand: what is a measurement?

What is a Measurement?

Measurement is the process of experimentally obtaining one or more quantity values that can reasonably be attributed to the item being measured. Measurements are considered a process because they typically take a series of actions or steps to obtain the final reported quantity value. Some examples of measurements include: measuring the length of a board, measuring the time it takes to travel from point A to point B, measuring the outside air temperature, measuring the mass (or weight) of an elephant, measuring the volume of milk in a milk jug, or measuring the percent cocoa in a chocolate candy bar.

Variation in the Measurement Process

Please note, no measurement is exact. It is merely a quantity value that represents the possible true value of the item being measured. If one were to take the same item and measure the item multiple times under the same conditions, a different quantity value would most likely be obtained. This variability between the quantity values relates to the variation in the measurement process and is due to various factors that influence the measurement process. Some of the factors that influence the measurement process and contribute to this variability include, but are not limited to, the measurement procedure, the skill and experience of the operator, the environmental conditions, and the measuring equipment.

Measurement Uncertainty in ISO/IEC 17025:2017

ISO/IEC 17025:2017 states that laboratories must identify the contributions to measurement uncertainty. All contributions that are of significance, including those from sampling, will be taken into account using appropriate methods of analysis. When estimating measurement uncertainty, the laboratory must take the time to thoroughly evaluate the accuracy of the measurements. The laboratory must consider all of the components that contribute to the variability of the measurement process and then estimate those contributions that significantly impact the measurement result. The laboratory must also use multiple statistical calculations to measure or estimate the variability in the measurement process. All of this culminates into an expanded measurement uncertainty. This expanded measurement uncertainty provides an estimate of the range of values that could reasonably represent the true quantity value within a stated coverage probability or level of confidence.

Reporting Measurement Uncertainty

What is done with this information? To report or not to report? First and foremost, all laboratories accredited to ISO/IEC 17025:2017 must provide accurate, clear, unambiguous, and objective results and include all information necessary for the interpretation of the results. Calibration laboratories must always include the measurement uncertainty on calibration certificates. However, for testing laboratories, they have a choice. The uncertainty must be included on the report when it is necessary for the interpretation of the results and when the measurement uncertainty is relevant to the validity or application of the test results, when the customer’s instruction requires it, or when the measurement uncertainty affects conformity to a specification limit. 

For either testing or calibration laboratories, when the measurement uncertainty is reported, it must be presented in the same unit as that of the measurand or a term relative to the measurand (e.g., percent). 

Why Report the Measurement Uncertainty?

So, what are the benefits to reporting the measurement uncertainty? 

Measurement uncertainty expresses the variability in the measurement process, qualifies the accuracy of the result, and provides the reader of the report with sufficient information to be able to interpret the measurement result to make an informed decision about what the measurement results represents. 

Reporting the measurement uncertainty provides meaning and understanding regarding the range of values that the measurement result represents and so the customer can interpret the result and make an informed decision.

Learn More About Measurement Uncertainty

You can find more information on ISO/IEC 17025 laboratory accreditation here. To learn more about ISO/IEC 17025:2017 measurement uncertainty requirements and evaluating and reporting measurement uncertainty, you register for an upcoming session of the ANAB training courses, Measurement Confidence: Fundamentals and Measurement Uncertainty: Practical Applications.

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