NIST Approach for Certification

The value has an assodated uncertainty that generally spedfies a range within which the true value is expected to lie at a level of confidence of approximately 95 % if the sample is homogeneous or the uncertainty represents a prediction interval within which the true values of 95 % of all samples are expected to lie at a stated level of confidence if significant sample heterogeneity is included. 

The three historical approaches to certification mentioned above were recently expanded to identily seven modes that are used at NIST for value assignment for chemical composition (May et al. 2000). These seven modes and the resulting values are summarized in Table 3.13. The basic principles of value assignment remain unchanged however, these modes now provide a well-defined link between the process used for value assignment and the definition of the assigned value (i.e. certified, reference, or information value). The terms described above provide a clear indication of the level of confidence that NIST has in the accuracy of the assigned value. The definition of a certified value implies that NIST must be involved in the measurement process for the value to be classified as a NIST certified value (see modes 1-3 in Table 3.13). Thus, modes 4 and 7, which do not involve NIST measure- 

Certification at NISTusing a primary method with 5i confirmation by other method(s) 

Certification/value assignment using one method at 5i NIST and different methods by outside collaborating laboratories 

Value assignment based on measurement of two or more outside collaborating laboratories using different methods 

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