Assignment of a Reference Value Independent from the Participants and Its Implications Regarding Performance Evaluation in Proficiency Testing on the Determination of Raw Milk Freezing Point

Abstract

Milk adulteration may cause losses or quality problems in the dairy industry, with implications regarding consumers’ health. The addition of water is one of the most common adulterations, which can be detected by determining the freezing point of milk. A proficiency testing (PT) program is in place in Chile concerning the determination of the freezing point of raw milk. A limited number of laboratories participate regularly to assure the validity of their measurements. The reference values were obtained independently through a calibration test, as recommended by ISO 13528:2022 in those cases where a small number of participants are evaluated. The uncertainty of the reference values assigned to nine PT rounds that included three PT items was investigated. A modification has been proposed for their calculations, which consists of the incorporation of the CRM uncertainty. This involves assessing the participants’ performance using the z’-score across all laboratories utilizing the thermistor method, which presents a slightly higher likelihood of inaccurate evaluation (2.9% of total participations). The assigned values demonstrated compatibility with the participants’ results. It is recommended that the same standard deviation for proficiency assessment be utilized for evaluating the performance of participants.