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Based on the theory of error propagation, the uncertainty in each comparison(measurement) accumulates in the uncertainty of the result. Each standardshould be supplied with a reference value and its uncertainty. Also, eachmeasuring operation (e.g. weighing of the sample, measuring the volume oftreated sample, recording the signal for a sample or standard) is subject to someuncertainty. consequently, the shorter the traceability chain is, the lower will bethe uncertainty of the result - with obvious exceptions. In principle, an absolutemethod will provide less uncertain results than win a relative method.-The "strength' of each link (comparison) in the chain depends on the qualityof the measurement operations and that of the standards used. It should beassured by means of internal quality controls and external evaluations (audits)of the laboratory concerned.one other implicit component of traceability of a result (Fig. 3.12) is thecomprehensive, documented "history'' of its generation (production). As shownin Fig.3.13, the history should include every element involved in the cMp andanswer the following questions: who carried out the cMp? what materials orinstruments were used? How and under which conditions was the cMp conducted?when were the operations involved performed? The history shouldcomprise not only the sample measurement process but also any calibrationoperations.Realizing the traceability of a result to base standards via solid, reliablecomparisons (path A in Fig. 3.14) involves (a) assuring traceability among thestandards in the unbroken measurement chain used by each laboratory (patn Bin Fig. 3.14) and (b) assuring consistent, reliable relationships among the resultsproduced by various laboratories (path c in Fig. 3.14). This latter condition hasa major practical implication as it ensures comparability among tre resultssupplied by different laboratories, which facilitates harmonization and mutualacceptance of their proficiency (a crucial ingredient of currently booming internationalsocial and trade relations).
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