152
8 Measurement Uncertainty
– Is an estimation for the changeability of conditions in different laboratories;
– Can be an indicator of the typical values of uncertainty (as an approximation).
Expressing the uncertainty: general remarks
– For low concentrations (close to the limit of determination), use absolute values—at this level, the uncertainty does not depend on the concentration;
– For high concentrations, use relative values—for high concentrations, the uncertainty is approximately proportional to the concentration.
The meaning of the uncertainty
The uncertainty budget comprises many components that influence, to a various
degree, the combined uncertainty. In laboratory practice, which of the components
have a significant influence on the combined uncertainty and which are neglected
can be assessed.
When constructing the uncertainty budget, it is recommended to carry out a preliminary assessment of all identified components, so as to select those whose contribution is most significant. It can be concluded that those components whose value is
not higher then one fifth of the biggest component will only have a 2% share in the
combined standard uncertainty.
BE AWARE! Always consider the number of components of the same kind. If they
are omitted individually, this can lead to underestimation of the combined uncertainty.
Uncertainty and decision-making
The knowledge on uncertainty values is essential in all situations where the result
is close to the decision limits; for example, the highest acceptable concentration
of a substance contaminating the sample or the lowest acceptable level of a given
component (e.g., biologically active substance in a pharmaceutical formulation). In
such cases, the value of the uncertainty assigned to the result can have a significant
meaning in decision-making, hence attempts are made to obtain the smallest possible
uncertainty. In practice, situations arise like the ones shown in Fig. 8.5.
Cases 1 and 4
The result of the measurement with the assigned uncertainty does not include the
decision value in its range. Assuming that the presented case concerns the highest
acceptable concentration of a given substance in the tested object, two scenarios can
be distinguished in which the decision-making body can make a clear-cut decision:
1. The result with the assigned uncertainty is above the permissible content.
The sample does not meet the requirements.
4. The result with the assigned uncertainty is below the permissible content.
The sample meets the requirements.
8 Measurement Uncertainty
– Is an estimation for the changeability of conditions in different laboratories;
– Can be an indicator of the typical values of uncertainty (as an approximation).
Expressing the uncertainty: general remarks
– For low concentrations (close to the limit of determination), use absolute values—at this level, the uncertainty does not depend on the concentration;
– For high concentrations, use relative values—for high concentrations, the uncertainty is approximately proportional to the concentration.
The meaning of the uncertainty
The uncertainty budget comprises many components that influence, to a various
degree, the combined uncertainty. In laboratory practice, which of the components
have a significant influence on the combined uncertainty and which are neglected
can be assessed.
When constructing the uncertainty budget, it is recommended to carry out a preliminary assessment of all identified components, so as to select those whose contribution is most significant. It can be concluded that those components whose value is
not higher then one fifth of the biggest component will only have a 2% share in the
combined standard uncertainty.
BE AWARE! Always consider the number of components of the same kind. If they
are omitted individually, this can lead to underestimation of the combined uncertainty.
Uncertainty and decision-making
The knowledge on uncertainty values is essential in all situations where the result
is close to the decision limits; for example, the highest acceptable concentration
of a substance contaminating the sample or the lowest acceptable level of a given
component (e.g., biologically active substance in a pharmaceutical formulation). In
such cases, the value of the uncertainty assigned to the result can have a significant
meaning in decision-making, hence attempts are made to obtain the smallest possible
uncertainty. In practice, situations arise like the ones shown in Fig. 8.5.
Cases 1 and 4
The result of the measurement with the assigned uncertainty does not include the
decision value in its range. Assuming that the presented case concerns the highest
acceptable concentration of a given substance in the tested object, two scenarios can
be distinguished in which the decision-making body can make a clear-cut decision:
1. The result with the assigned uncertainty is above the permissible content.
The sample does not meet the requirements.
4. The result with the assigned uncertainty is below the permissible content.
The sample meets the requirements.