Measurement Error Uncertainty and Tolerance: Why These Concepts Should Not Be Confused

This working paper explains the differences among measurement error, measurement uncertainty, and tolerance—three closely related concepts that are frequently confused in calibration, testing, manufacturing, and conformity assessment. Measurement error describes the observed difference between a measured value and a reference quantity value. Measurement uncertainty characterizes the dispersion of values that can reasonably be attributed to the measurand. Tolerance defines the permitted limits established by a specification, standard, manufacturer, regulator, or user. The paper discusses how these concepts interact in calibration and conformity assessment, with particular attention to decision rules, guard bands, false acceptance, and false rejection. Illustrative examples involving defibrillator energy, infusion pump flow rate, and non-invasive blood pressure measurement demonstrate why a measured value alone may not always provide sufficient evidence for a defensible conformity decision. The numerical limits used in the examples are provided solely to explain the decision-making process and should not be interpreted as requirements from a particular medical-device standard or manufacturer.