Measurement is a process to determine the value or physical quantity of a measurand. The measurand is the particular quantity subject to measurement [1]. Each test of dimensional measurement contains one or more measurement steps. These steps contain inaccuracies or errors. To describe these errors the term "Uncertainty" is used. The ISO International Vocabulary of Basic and General Terms in Metrology (VIM) define uncertainties as "a parameter, associated with the result of a measurement that characterizes the dispersion of the values that could be reasonably attributed to the measurand (VIM 3.9)." In general, different measurements may have different uncertainty statements, even if they are measured with the same instrument [2]. Elimination of uncertainty is not the focus for measurement only the reduction of it is possible. The most common dimensional measurand is associated with the work piece. The example can be the measurand when measuring the diameter of the hole by using micrometer or any other instrument. The uncertainty associated with this kind of measurand is known as "Task specific uncertainty". The uncertainty associated when testing a piece of equipment is a new concept in this field. As a result this discussion focuses on this new type of uncertainty which is known as "Test uncertainty". The example can be to calibrate micrometer by using a gauge block or to calibrate CMM by using a step gauge. The uncertainty associated with this kind of measurand which is the "error of the instruments" is known as "Test Uncertainty". This is mentioned in ISO 23165 in which test uncertainty is characterized for CMM.
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