Each measured result is related to measurement uncertainty. In order for the results to be useful, reliability needs to be at a sufficient level of measurement uncertainty that is also taken into account in the reporting of results.
A measuring result is always impacted by several factors that we can never know for certain. Thus, all measuring results include a specific uncertainty, range, that describes the assumed variation of the measuring result.
Measurement uncertainty is required for the assessment of result reliability, for the comparison of results, to prove conformity, for mutual assessment of methods and to create traceability.
Identification, assessment and, if possible, calculation of factors impacting the measurement uncertainty helps to understand factors and mechanisms of action influencing the result. Knowledge of measurement uncertainty creates a basis for developing measurements to the level necessary.
There are instructions and guidebooks on the specification of measurement uncertainty, including the following on the right column.