Proficiency testing (PT) plays a crucial role in assessing the competence and performance of laboratories by subjecting them to interlaboratory comparisons. This paper focuses on evaluation and comparison of approaches for determining the standard deviation for proficiency assessment ( \(\text{SDPA or}\; {\sigma }_{{\text{PT}}}\) ) used for establishing performance criteria in PT schemes. It explores options provided in ISO 13528:2022 that utilize laboratory method precision, information from previous PT rounds, regulatory or expert set acceptance criteria, criteria based on a general model for repeatability or reproducibility, and data from the same round of PT for setting evaluation criteria. The advantages and disadvantages of each option are examined alongside practical examples of their application. The insights into the various strategies to establish SDPA provided in this paper are aimed to help laboratories, and PT providers with the selection of appropriate criteria that align with their specific needs and objectives and ensure a fit-for-purpose performance assessment that enhances comparability among laboratories, and promotes continual quality improvement in PT programs.
