For an accredited laboratory, the best measurement capability for a particular quantity is “the smallest uncertainty of measurement a laboratory can achieve within its scope of accreditation when performing more or less routine calibrations of nearly ideal measurement standards  intended to define, realize, conserve, or reproduce a unit of that quantity or one or more of its values; or when performing more-or-less routine
calibrations of nearly ideal measuring instruments designed for the measurement of that quantity.” (EA-4/02) The best measurement capability is based on evaluations of actual measurements using generally accepted methods of evaluating measurement uncertainty.

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