Budgeting under Uncertainty

In their broadest application, CRMs are used as controls to verify in a direct comparison the accuracy of the results of a particular measurement parallel with this verification, traceability may be demonstrated. Under conditions demonstrated to be equal for sample and CRM, agreement of results, e.g. as defined above, is proof. Since such possibilities for a direct comparison between samples and a CRM are rare, the user s claims for accuracy and traceability have to be made by inference. Naturally, the use of several CRMs of similar matrix but different analyte content will strengthen the user s inference. Even so, the user stiU has to assess and account for all uncertainties in this comparison of results. These imcertainty calculations must include beyond the common analytical uncertainty budget (i) a component that reflects material matrix effects, (2) a component that reflects differences in the amount of substance determined, (3) the uncertainty of the certified or reference value(s) used, and 4) the uncertainty of the comparison itself AU this information certainly supports the assertion of accuracy in relation to the CRM. However, the requirement of the imbroken chain of comparisons wiU not be formally fulfilled. 

Precision is a measure of the variability of results obtained under different circumstances (e.g., repeatability or reproducibility). It is usually expressed as a standard deviation. Uncertainty is a general concept that covers all aspects of our lack of knowledge of the true value. It is assessed by an uncertainty budget and also is expressed in terms of a standard deviation. (Sections 1.7, 1.8)... 

Measurements under a fixed real count time may suffer from an increased uncertainty when count losses exceed 20%. The increase could be a significant extra contribution to an uncertainty budget. When nsing a loss-free counting system, artificial counts are added to the spectrum to compensate for lost counts in a manner that is not random. That distorts the statistics to a small extent. 

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