Fundamentals in Measurement: The Role of Measurement Uncertainty in Conformity Assessment: Some Examples

The statements of conformity are widely spread in testing, inspection, and calibration fields and are essential to ensure that products, materials, services, and systems comply with established standards, regulations, and legal requirements. In calibration, this activity consists of the comparison of the measurement results within the defined limits (e.g., specifications). This task can be very challenging when the measurement result is close to the limit, making it difficult to take a decision on compliance (or noncompliance) with respect to the limits. Measurement uncertainty introduces the risk that the actual value of the measurand is out of specification even if the measured value falls within the specification limits, leading to the Probability of False Acceptance (PFA), or is in specification even if the measured value falls outside, causing the Probability of False Rejection (PFR). The way in which measurement uncertainty is taken into account when declaring compliance with a specific requirement is called “decision rule” and is defined according to the risk that researchers are willing to accept. In general, the decision rule can take measurement uncertainty into account either directly, by guard banding, or indirectly, such as by imposing an upper limit on measurement uncertainty (e.g., through defining the ratio between specification and measurement uncertainty).