= Reference material homogeneity
\b[Reference material homogeneity] concerns whether variation of specified properties within or between usable portions is small enough for the intended measurement purpose. Homogeneity is property-specific and depends on the portion taken: uniform colour need not establish uniform <refractive index>, composition or another measured property. Sufficient homogeneity is part of the definition of a <reference material>.<International vocabulary of metrology by JCGM>[VIM3entry5.13, definition]{ref}
An original study takes several bottles from a batch and measures multiple portions from each, with the measurement order varied to avoid confusing bottle order with <instrumental drift>. A simple <measurement model> is
$$
y_{ij}=\mu+b_i+e_{ij},
$$
where $b_i$ describes bottle-to-bottle variation and $e_{ij}$ includes variation among sampled portions and measurement noise. For independent zero-mean contributions, the <variance> of a single observation is
$$
\operatorname{Var}(y_{ij})=\operatorname{Var}(b_i)+\operatorname{Var}(e_{ij}).
$$
Repeatedly reading one portion mainly probes repeat measurement scatter. It does not replace sampling different portions or bottles. Separating material variation from <measurement repeatability> requires a sampling and replication design appropriate to both.
Suppose an original fitted model gives a between-bottle standard deviation $3\times10^{-6}$ in <refractive index> and within-bottle measurement scatter $4\times10^{-6}$. Their combined standard deviation for one observation is $5\times10^{-6}$ under the independence assumption. Averaging more readings from the same bottle can reduce the second contribution, but does not average away that bottle's common $b_i$ contribution. These are illustrative model values, not certificate data.
NIST's archived SRM1922 characterization tested multiple oil portions using minimum-deviation measurements and an Abbé-type <refractometer>.<Certificate for Standard Reference Material 1922 by National Institute of Standards and Technology>[2009certificate, p1, Certified Values and Uncertainties, homogeneity tests on multiple aliquots]{ref} Sampling different portions answers the homogeneity question directly; <reference material stability> separately addresses changes over time. A report should retain the sampled portion size and conditions so that a user can judge whether the evidence applies to their own use.
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