In eCerto: Statistical Tests for the Production of Reference Materials

LTS statistics

The Tool calculates the expected life time of a reference material in several steps:

1. calculating a linear model for the measurement data $y = b_1 \times x + b_2$, where $y$ represents measured values and $x$ represents time (expressed in month)
2. correcting the intercept $b_2$ for the difference between the mean obtained from LTS data and the mean reported as certified value on import $b_2' = b_2 + \mu_\mathit{LTS} - \mu_c$
3. using the corrected $b_2'$ and $b_1$ to estimate the time point when the value of the certified analyte is expected to exceed the interval of $\mu_c \pm U$

The calculation results are depicted in Fig.L1 and can be exported as a report in PDF format.

Note! The $U$ defining the interval around $\mu_c$, which we expect the property values to remain in within the RM life time, is taken from the data read upon initial Excel import. The user should be careful regarding the value specified here to avoid overestimating the life time. LTS monitoring, which is usually performed within the same lab, will cover mostly the uncertainty due to stability of a material property. The uncertainty defined in the original certificate will cover additional uncertainty contributions (i.e. from the collaborative trial). Hence, it might be adequate to use only a fraction of the certified $U$ value to define the interval.

Note! The parameters of a linear model, i.e. $b_1$, can only be determined with some uncertainty. While the current report layout calculates the life time based on $b_1$ as described above, a more conservative estimate would be to use the confidence interval, $CI_{95}(b_1)$, instead. Calculation based on $CI_{95}(b_1)$ is shown in Fig.L1 by default.

eCerto documentation built on April 12, 2025, 9:13 a.m.