The initial publication of the Uncertainty in Measurement or GUM relates to the documents that would be resulted in a worldwide awareness of the importance of a realistic estimate of the value reported. The evaluation of uncertainty in measurement is introduced by the GUM bottom up and top down approaches is derived from the principles applied in physical measurements. Many testing laboratories have already experienced the large problems in applying these principles for chemical measurements, resulting in time-consuming evaluations and costly additional experiments.
Other approaches of GUM have been proposed to obtain a realistic estimate of the range in which the true value of the measurement may be found with a certain degree of probability. Among different approaches, the top down approach of GUM is based on the standard deviation in the results of inter comparison data. This approach is much easier for tests for which it is either difficult to establish a full measurement equation. The bottom up approach for evaluating the uncertainty in measurement can easily be applied in instrumental neutron activation analysis.
The significant sources of GUM bottom up and top down approaches is used for evaluating the uncertainty. Valuable techniques are used in the evaluation of test validity approaches. In this contribution, the top-down evaluation of uncertainty derived from the participation of inter comparison rounds and proficiency testing schemes will be presented. Here, the results will be compared with the bottom-up evaluation of uncertainty, and ease of applicability, validity and usefulness of both approaches will be discussed.