9.10 Control Charts
167
15%. It follows that both components—both the systematic component of 3% and
random component of 4% of the recovery testing—can be included in the uncertainty
budget. The complex uncertainty of the recovery is u
√
32 + 42 5%.
Case 2: the recovery component is included in the uncertainty budget; the systematic component is used to introduce the recovery factor.
The content of the pesticide p, p
-DDE in the sausage has been determined. On
the basis of the test of enriched samples (after they were minced), the recovery value
was determined at the level of 60% + 8%. The value of the measurement uncertainty,
stemming from the requirements of the food safety agency, has been determined at the
level of 20%. It follows that including the systematic component (with a result 40%
lower than the expected value), would cause the exceeding of the required value
of uncertainty of 20%. With regards to that, the laboratory has used the recovery
factor—that is, all the measurement results for real samples have been multiplied by
the 1.6 factor and the variability of the recovery value of 8% was included in the
uncertainty budget.
9.10.4 Range Control Chart
Another type of control chart is the range chart, on which the difference values
between results for the two portions of the same control sample (most often analyzed
in parallel) are recorded.
NOTE! If the program of internal quality management only provides for the
evaluation of the results of parallel measurements of two sub-samples, then there is
a danger that systematic errors will not be noticed.
The range chart is a one-sided card, which means that on the chart there is a
determined line of the mean value for the calculated difference of two results and
two upper lines of warning and action. The range chart is especially useful in the case
of conducting tests for laboratory samples with a variable content of the analyte or
with a changeable matrix content, and for non-durable samples in which the analyte
concentration is rapidly changing.
9.11 How to Evaluate Control Charts
Interpretation of control charts is based on the assumption that the estimators of
specific subsets (i.e. series of samples)—for example, the mean value—are changing
randomly, rarely exceeding the control limits. The chart is divided into six areas. each
1 s wide, where s is the estimated standard deviation value of the test collective. The
areas are placed symmetrically around the central line.
In cases where all points are between the internal control lines, the stability of the
measuring system is assumed. However, if the points are more often on one side of
167
15%. It follows that both components—both the systematic component of 3% and
random component of 4% of the recovery testing—can be included in the uncertainty
budget. The complex uncertainty of the recovery is u
√
32 + 42 5%.
Case 2: the recovery component is included in the uncertainty budget; the systematic component is used to introduce the recovery factor.
The content of the pesticide p, p
-DDE in the sausage has been determined. On
the basis of the test of enriched samples (after they were minced), the recovery value
was determined at the level of 60% + 8%. The value of the measurement uncertainty,
stemming from the requirements of the food safety agency, has been determined at the
level of 20%. It follows that including the systematic component (with a result 40%
lower than the expected value), would cause the exceeding of the required value
of uncertainty of 20%. With regards to that, the laboratory has used the recovery
factor—that is, all the measurement results for real samples have been multiplied by
the 1.6 factor and the variability of the recovery value of 8% was included in the
uncertainty budget.
9.10.4 Range Control Chart
Another type of control chart is the range chart, on which the difference values
between results for the two portions of the same control sample (most often analyzed
in parallel) are recorded.
NOTE! If the program of internal quality management only provides for the
evaluation of the results of parallel measurements of two sub-samples, then there is
a danger that systematic errors will not be noticed.
The range chart is a one-sided card, which means that on the chart there is a
determined line of the mean value for the calculated difference of two results and
two upper lines of warning and action. The range chart is especially useful in the case
of conducting tests for laboratory samples with a variable content of the analyte or
with a changeable matrix content, and for non-durable samples in which the analyte
concentration is rapidly changing.
9.11 How to Evaluate Control Charts
Interpretation of control charts is based on the assumption that the estimators of
specific subsets (i.e. series of samples)—for example, the mean value—are changing
randomly, rarely exceeding the control limits. The chart is divided into six areas. each
1 s wide, where s is the estimated standard deviation value of the test collective. The
areas are placed symmetrically around the central line.
In cases where all points are between the internal control lines, the stability of the
measuring system is assumed. However, if the points are more often on one side of