= Measurement standard
A <measurement standard> realizes a specified quantity with an assigned value and associated <measurement uncertainty>, and is used as a reference. It can be a physical artifact, a measuring system or a suitable reference material. The value and its <measurement uncertainty> apply under stated conditions and at the relevant time; a nominal label alone is insufficient.<International vocabulary of metrology by JCGM>[VIM3 entry5.1, definition and notes1–5]{ref}
For an original comparison, take a reference <resistor> with assigned <electrical resistance> $R_s=100.000\,\Omega$ and <standard uncertainty> $u(R_s)=0.001\,\Omega$. A comparison measures a working resistor's ratio $q=R_x/R_s=1.00020$, with independent $u(q)=0.00001$. The <measurement model> gives
$$
R_x=qR_s=100.020\,\Omega,
$$
and first-order <propagation of uncertainty> gives
$$
u(R_x)=\sqrt{q^2u^2(R_s)+R_s^2u^2(q)}\simeq0.0014\,\Omega.
$$
The reference's <measurement uncertainty> remains in the result even if the ratio could be read with arbitrarily many digits.<Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results by Barry N. Taylor and Chris E. Kuyatt>[AppendixA, equationA-3]{ref}
A practical comparison records reference conditions, connecting arrangements and repeated readings while checking effects such as <instrumental drift>. A <primary measurement standard> has a particular basis for its assigned value; a working reference need not be primary to transfer useful <metrological traceability>. Distinguish this physical reference from a written technical specification, which is another use of the word standard.
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