It can be seen from the definition that the uncertainty is the degree of dispersion of the characterization result for the measurement result, so it can be described by a quantitative number, that is, it is a quantitative concept. For standard materials, the uncertainty of the fixed value results consists of three parts: the uncertainty caused by the non-uniformity of the standard substance, the uncertainty caused by the instability of the standard substance, and the determination of the reference substance. The uncertainty caused by the value process. The uncertainty of the fixed value uncertainty and the uniformity test and the stability test are superimposed according to the square and square method to give the synthetic standard uncertainty, which is denoted as uc. The uncertainty of the synthetic standard uncertainty multiplied by the factor (this factor is called the inclusion factor) is called the extended uncertainty or the total uncertainty, denoted as U. When giving extended uncertainty, a value containing the factor (denoted as k) should be specified, which is related to the required confidence probability and degree of freedom.
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