### 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 $\mathit{SK}$.
