Evaluation of Measurement Uncertainty
Precision and accuracy are of prime interest when analyzing quality in a measurement procedure. Precision provides information about the dispersion of the measured values with respect to the true value of the measurand. It describes an internal quality of the measurement process. In general, it is expressed by means of the uncertainty, which is a non-negative parameter characterizing the dispersion of the quantity values being attributed to the measurand. On the other hand, the term accuracy may only be used in comparison to reference values of the same measurand. Accuracy is a term relating the mean of the measurements to the reference value, while precision is representative of the spread of these measurements. Perfection in measurement can never be achieved since even a precise and accurate measurement will have some remaining uncertainty. It is best to use the term accuracy only as a quantitative term, or for broad comparisons between methods, and use the term uncertainty as the qualitative assessment. When the uncertainty of measurement is evaluated and stated, then the fitness of purpose for a particular application can be properly understood. In the present Chapter the discussion is focused on the principal techniques for uncertainty estimation in Large-Scale Dimensional Metrology applications. Practical examples referring to MScMS-I and MScMS-II are presented and discussed as well.