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9 Managing the Quality Systems
The placement of the border warning line and the action line can be determined:
– On the basis of a set of results and their statistical evaluation;
– Arbitrarily on the basis of legal requirements.
Shewhart’s control charts are used for the ongoing control of the inter-laboratory
variability of results of measurements conducted with the help of the same measuring procedure and for the same control sample. Creating a control chart requires
determining the placement of the central line and the placement of the warning (2)
and action (3) lines.
9.10.2 Shewhart’s Chart
Figure 9.1 shows an example control chart of the Shewhart type. The control chart
usually holds 20 or, less often, 40 measurement results. The nominal value is placed
on the chart before the start of the measuring series.
In the case when the value for the central line is not an arbitrarily accepted reference value (nominal, normative), the mean value and the standard deviation are
calculated after the first control chart for a given control sample has been filled.
9.10.3 Recovery Control
Control charts can also be used to evaluate the recovery when a known amount of
the RM is added to the real samples. In an ideal scenario, the recovery rate should
be 100%, whereas the variability of results obtained for subsequent measurements
(standard deviation) should be a component of the uncertainty budget. In reality,
it often happens that the recovery is smaller or bigger (depending on the type of
interference) than 100%; in such situations, the information obtained on the basis
of the gathered results allows the recovery value (the systematic component) to be
evaluated in addition to evaluation of the results spanning the mean value of recovery
(the random component). Both of those values must be included when giving the
result.
The variability of results is always a component of the uncertainty budget, whereas
the method load can be a component of the uncertainty budget or it can be included
in the calculation in the form of a recovery factor.
Example
Case 1: both components (systematic and random) are included in the uncertainty
budget.
The content of methylmercury in tuna tissue has been determined. On the basis
of the test of enriched samples (after their incubation), the recovery value was determined at the level of 97% + 4%. The value of the measurement uncertainty, stemming
from the requirements of the food safety agency, has been determined at the level of
9 Managing the Quality Systems
The placement of the border warning line and the action line can be determined:
– On the basis of a set of results and their statistical evaluation;
– Arbitrarily on the basis of legal requirements.
Shewhart’s control charts are used for the ongoing control of the inter-laboratory
variability of results of measurements conducted with the help of the same measuring procedure and for the same control sample. Creating a control chart requires
determining the placement of the central line and the placement of the warning (2)
and action (3) lines.
9.10.2 Shewhart’s Chart
Figure 9.1 shows an example control chart of the Shewhart type. The control chart
usually holds 20 or, less often, 40 measurement results. The nominal value is placed
on the chart before the start of the measuring series.
In the case when the value for the central line is not an arbitrarily accepted reference value (nominal, normative), the mean value and the standard deviation are
calculated after the first control chart for a given control sample has been filled.
9.10.3 Recovery Control
Control charts can also be used to evaluate the recovery when a known amount of
the RM is added to the real samples. In an ideal scenario, the recovery rate should
be 100%, whereas the variability of results obtained for subsequent measurements
(standard deviation) should be a component of the uncertainty budget. In reality,
it often happens that the recovery is smaller or bigger (depending on the type of
interference) than 100%; in such situations, the information obtained on the basis
of the gathered results allows the recovery value (the systematic component) to be
evaluated in addition to evaluation of the results spanning the mean value of recovery
(the random component). Both of those values must be included when giving the
result.
The variability of results is always a component of the uncertainty budget, whereas
the method load can be a component of the uncertainty budget or it can be included
in the calculation in the form of a recovery factor.
Example
Case 1: both components (systematic and random) are included in the uncertainty
budget.
The content of methylmercury in tuna tissue has been determined. On the basis
of the test of enriched samples (after their incubation), the recovery value was determined at the level of 97% + 4%. The value of the measurement uncertainty, stemming
from the requirements of the food safety agency, has been determined at the level of