D.4.1 The approach to evaluating and expressing the uncertainty of a measurement result on which the NIST policy and this Technical Note are based is applicable to evaluating and expressing the uncertainty of the estimated value of a measurand that is defined by a standard method of measurement. In this case, the uncertainty depends not only on the repeatability and reproducibility of the measurement results (see subsections D.1.1.2 and D.1.1.3), but also on how well one believes the standard measurement method has been implemented. (See example H.6 of the Guide.)
When reporting the estimated value and uncertainty of such a measurand, one should always make clear that the measurand is defined by a particular method of measurement and indicate what that method is. One should also give the measurand a name which indicates that it is defined by a measurement method, for example, by adding a modifier such as "conventional." (See also subsection D.6.1)
D.4.2 There are national as well as international standards that discuss the characterization of test methods by interlaboratory comparisons. Execution of test methods according to these standards, both in the characterization stage and in subsequent measurement programs, often calls for the expression of uncertainties in terms of defined measures of repeatability and reproducibility. When NIST authors participate in such characterization or measurement programs, NIST policy allows for the results to be expressed as required by the relevant standards (see Appendix C, section 4). However, when NIST authors document work according to such standards, they should consider making the resulting publication understandable to a broad audience. This might be achieved in part by giving definitions of the terms used, perhaps in a footnote. If possible, NIST authors should relate these terms to those of this Technical Note and of the Guide.
If a test method is employed at NIST to obtain measurement results for reasons other than those described above, it is expected that the uncertainties of these measurement results will be evaluated and reported according to section 2 of the NIST policy (see Appendix C). This would be the case, for example, if measurement results from a characterized test method are compared to those from a new method of measurement which has not been characterized by interlaboratory comparisons.
D.4.3 When an unknown standard is calibrated in terms of a known reference standard at lower levels of the measurement hierarchy, the uncertainty of the result of calibration may have as few as two components: a single Type A standard uncertainty evaluated from the pooled experimental standard deviation that characterizes the calibration process; and a single Type B (or possibly Type A) standard uncertainty obtained from the calibration certificate of the known reference standard.
NOTE - The possibility of unsuspected systematic effects in the calibration process used to calibrate the unknown standard should, however, not be overlooked.