Although the combined standard uncertainty u c ( y ) can be sufficient to express the uncertainty of a measurement result, it is almost always necessary to provide a measurement of the uncertainty which will define, around the measurement result, an interval in which we will be able to find a large part of the distribution of values that could reasonably be attributed to the measurand.
In general, the measurement results are distributed around an average value m which is the measurand's most probable estimate. We can define an interval such as the probability where is called confidence threshold and confidence level. We have therefore defined a confidence interval of width ±U around the measurand's average value. U is obtained by multiplying the combined standard uncertainty uc(y) by a coverage factor k such as is called expanded uncertainty.