Expanded Uncertainty (U)

Quantity defining an interval about a result of a measurement that may be expected to encompass a large fraction of the distribution of values that could reasonably be attributed to the measurand. DISCUSSION(1) The fraction may be regarded as the coverage probability or level of confidence of the interval. (2) To associate a specific level of confidence with the interval defined by the expanded uncertainty requires explicit or implicit assumptions regarding the probability distribution characterized by the measurement result and its combined standard uncertainty. The level of confidence that may be attributed to this interval can be known only to the extent to which such assumptions can be justified. (3) An expanded uncertainty U is calculated from a combined standard uncertainty uc and coverage factor k using: U =k X uc.