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Representativeness;
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Completeness;
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Comparability; and
•
Sensitivity
Accuracy and precision are monitored through the analysis of QC samples. Completeness is a calculated
value. Sensitivity is monitored through instrument calibration and the determination of method detection
limits (MDLs) and reporting limits. Qualitative quality objectives, expressed in terms of comparability
and representativeness, are addressed as part of the sampling design.
Accuracy
Accuracy is a measure of the overall agreement between an observed value and an accepted reference
value. Accuracy includes a combination of random error (precision) and systematic error (bias)
components that are due to sampling and analytical operations. Accuracy is measured through the
analysis of matrix spikes and/or laboratory control samples, as and if required by the analytical methods,
to determine percent recoveries (%R). A certified reference standard is used if available.
The %R utilizing matrix spikes is calculated as follows:
%R = (C
s
– C
u
) x 100
C
a
where C
s
= measured concentration of spiked sample
C
u
= measured concentration of unspiked sample
C
a
= actual concentration of spike added
The %R utilizing laboratory control samples is calculated as follows:
%R = (C
m
) x 100
(C
a
)
where C
m
= measured concentration of control sample
C
a
= actual concentration of control sample
Accuracy should be assessed using a minimum of 6 determinations over a minimum of 3 concentration
levels (e.g. 3 concentrations/ 2 replicates) in a representative pool of sample matrix (preferably the same
pool of matrix used to prepare matrix controls). The analyte concentrations tested should be targeted to
the same region of the standard curve as the matrix controls. In general, accuracy should be within the
range of 70 - 130 percent recovery of exogenous analyte.
Precision
Precision is the degree to which a set of observations or measurements of the same property, obtained
under similar conditions, conform to themselves.