In eCerto: Statistical Tests for the Production of Reference Materials

Post certification

To check if the certified value of a CRM $\mu_c$ remains stable, a post certification measurement can be performed and evaluated according to ISO GUIDE 35:2017 (8.10.3.2) by calculating a stability criteria $\mathit{SK}$ using formula:

$$\mathit{SK} = {|\mu_c - \mu_m| \over \sqrt{u_c^2 + u_m^2}}$$

where $\mu_m$ is the mean and $u_m = \sigma / \sqrt{N}$ is the standard uncertainty of post certification measurements performed on a CRM specimen, and where $u_c = u_{com} \times \mu_c$ is the absolute certified uncertainty without expansion.

If $\mathit{SK} \le k$ is fulfilled, this indicates that the material is sufficiently stable.

Note! $k$ is set based on the expansion factor associated with the analyte given in Tab.C3. It is recommended to keep $k=2$ as this will ensure a 95% confidence for $\mathit{SK}$.

eCerto documentation built on April 12, 2025, 9:13 a.m.