In metrology, the estimation of the dispersion of the values “attributable” to the measurand is defined as the uncertainty of a measure. Measurement uncertainty is the degree of uncertainty with which the value of a physical quantity or property is obtained through its direct or indirect measurement. The result of the measurement is therefore not a single value, but a set of values derived from the measurement (direct or indirect) of the physical size or property itself. It is associated with the measurement value as follows in the example (a measurement of a diameter):
Uncertainty means the range of possible values within which the true value of the measurement lies. This definition changes the usage of some other commonly used terms. For example, the term accuracy is often used to mean the difference between a measured result and the actual or true value. Since the true value of a measurement is usually not known, the accuracy of a measurement is usually not known either. Because of these definitions, we modified how we report lab results. For example, when students report results of lab measurements, they do not calculate a percent error between their results and the actual value. Instead, they determine whether the accepted value falls within the range of uncertainty of their result.