Is the method used to assess uncertainty described in the GUM the only valid method?
The GUM is considered as the standard document for the assessment of uncertainty. Other approaches have been encouraged in international texts, such as ISO/IEC 17025 or Guide ILAC G17: 2002 "Introducing the concept of uncertainty of measurement in testing in association with the application of the standard ISO / IEC 17025", in which the basis for the estimation of uncertainty of measurement is to use existing knowledge. Existing experimental data should be used (quality control chart, validation, round robin test, Proficiency Testing, handbooks etc.[…]