ILAC Proficiency Testing Consultative Group, Task Force on H&S testing:

This discussion document is intended to present the technical issues concerning the homogeneity and stability of proficiency test samples. It is intended also to lead to consensus recommendations concerning the resolution of these issues. These recommendations have served as guidance for the revisions of ILAC G13 and ISO/IEC Guide 43 (ISO/IEC 17043) and for use by ILAC member bodies in assessing the suitability of PT schemes used by accredited laboratories. International guidelines for the accreditation of providers of proficiency testing or external quality assessment schemes (PT) require providers to demonstrate both the homogeneity and the stability of proficiency testing samples. This is essential if PT is to serve its purpose and be able to identify errors i