## 3 Results and Discussion
### Standard Uncertainty of the Assigned Values
#### 3.2.2 Uncertainty Components of the Assigned Values
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Standard** & **Standard** & **Standard** & **Combined Standard** \\
**PT Identification** & \begin{tabular}{c} **Uncertainty from** \\ **CRM Certificate** \\ **\(\mu_{CRM}\)** \\ **\(\mu_{CRM}\)** \\ **\((\text{m}^{\circ}\text{C})\)** \\ \end{tabular} & \begin{tabular}{c} **Uncertainty of the** \\ **CRM Analysis** \\ **\(u^{-}\)** \\ **\(\text{x}_{CRM}\)** \\ **\((\text{m}^{\circ}\text{C})\)** \\ \end{tabular} & \
\begin{tabular}{c} **UT Item** \\ **\(u^{-}\)** \\ **\(\text{x}_{SPI}\)** \\ **\((\text{m}^{\circ}\text{C})\)** \\ \end{tabular} &
\begin{tabular}{c} **Uncertainty of the** \\ **\(\text{m}^{\circ}\text{C}\)** \\ **\((\text{m}^{\circ}\text{C})\)** \\ \end{tabular} \\ \hline PC LC 1701 & 0.92 & 0.33 & 0.62 & 1.16 \\ PC LC 1702 & 0.92 & 0.29 & 1.14 & 1.49 \\ PC LC 1801 & 0.92 & 0.33 & 0.71 & 1.21 \\ PC LC 1802 & 0.92 & 0.29 & 0.72 & 1.20 \\ PC LC 1901 & 0.92 & 0.24 & 0.71 & 1.19 \\ PC LC 1902 & 0.92 & 0.33 & 0.61 & 1.15 \\ PC LC 2001 & 0.92 & 0.29 & 0.73 & 1.21 \\ PC LC 2002 & 0.92 & 0.29 & 0.76 & 1.23 \\ PC LC 2101 & 0.92 & 0.67 & 0.88 & 1.44 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Combined standard uncertainty of the value assigned (and its components) to the freezing point of raw milk in the PT rounds conducted between 2017 and 2021.  
become 0.94, which is very similar to the 0.96 factor previously mentioned, while with uncertainty of 0.5\(\hat{\sigma}\), the factor would become 0.88. Taking these factors into account, the risk of misevaluating the participants would only appear in edge cases, or close to the borders, which means \(\mid\)z-score \(\mid\) = 2 (it must be noted that when 2 \(<\) \(\mid\) z \(\mid\) \(<\) 3, performance is considered questionable) or \(\mid\)z-score \(\mid\) = 3 (it must be noted that z \(\leq\) 1 3 \(\mid\) is considered an unsatisfactory performance), where the z'-score value would be lower than the z-score and could alter the participant's evaluation. Of the total number of PT items evaluated (273) using the thermistor cryoscope method throughout all the rounds (96 participations and three PT items), the total number of performance evaluations that would change from questionable to satisfactory when using the z'-score would be 5 (1.8%), while those that would change from an evaluation of unsatisfactory to questionable would be 3 (1.1%), with the total percentage being 2.9% (8 changes). In the above cases, the uncertainty values correspond to 0.4\(\hat{\sigma}\) or 0.5\(\hat{\sigma}\), being \(\hat{\sigma}\) = 3 m\({}^{\circ}\)C.  
On the other hand, it should be taken into account that the laboratory that performed the analyses of the PT items to assign the reference value used the same analytical method as the participants and the standard deviation for proficiency assessment is based on the precision indexes of the method. Therefore, the compliance with the criterion \(u_{\hat{\pi}}\) \(<\) 0.3\(\hat{\sigma}\) depends on the sources of uncertainty already mentioned, the main one being that of the CRM certificate. These particular situations are not mentioned in the ISO 13528:2022 standard [13], while different approaches are accepted, both for the assignment of reference values as well as for the performance evaluation criterion. It also mentions that, generally, for scenarios where the assigned value and/or the standard deviation for the evaluation of the proficiency are not derived from the results of the participants, the use of z'-score may be preferred.