= Certified reference material
A \b[certified reference material] is a <reference material> supplied with authoritative documentation assigning specified property values and their associated uncertainties and traceabilities through valid procedures. The certification applies to the stated properties and conditions. It does not make every conceivable property of the material a certified value.<International vocabulary of metrology by JCGM>[VIM3entry5.14, definition and notes1,3–4]{ref}
A usable certificate identifies the material and relevant property, its assigned value, the uncertainty convention, conditions of use and the evidence needed for <metrological traceability>. Both <reference material homogeneity> and <reference material stability> matter when a portion is used later. The VIM distinguishes reference materials suitable for precision control from those with assigned values suitable for <measurement calibration> or <measurement trueness> control.<International vocabulary of metrology by JCGM>[VIM3entry5.13, note2]{ref}
For a physical example, the archived NIST SRM1922 mineral-oil certificate gives <refractive index> $n=1.46945$ at $589.3\,\mathrm{nm}$ and $20\,{}^{\circ}\mathrm C$, with <expanded uncertainty> $U=6\times10^{-5}$ and <coverage factor> $k=2$.<Certificate for Standard Reference Material 1922 by National Institute of Standards and Technology>[2009certificate, p1, uncertainty convention; p2, Table1, fitted589.3nm entry]{ref} Its corresponding <standard uncertainty> is $u_{\rm ref}=U/k=3\times10^{-5}$.
For an original comparison under those same conditions, suppose a <refractometer> gives $1.46953$ with independent <standard uncertainty> $u_{\rm inst}=4\times10^{-5}$. The discrepancy is $8\times10^{-5}$, and its combined <standard uncertainty> is
$$
u_\Delta=\sqrt{u_{\rm ref}^2+u_{\rm inst}^2}=5\times10^{-5}.
$$
Thus the discrepancy is $1.6u_\Delta$. This calculation is a diagnostic comparison, not an automatic pass/fail rule. Correlated contributions, a different temperature or an inappropriate sample invalidate the simple comparison model. The certificate supplies a documented reference and its limits; the user's complete <measurement result> still includes the measuring instrument and procedure.
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