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The precision of field samples is assessed by the comparison of field duplicates. The relative percent
difference (RPD) between the analyte levels measured in the field duplicates is calculated as follows:
RPD = | C
a
- C
b
| x 100
0.5(C
a
- C
b
)
where C
a
= measured concentration of sample
C
b
= measured concentration of duplicate sample
The precision of physical parameter readings may be assessed by the comparison of each instrument’s
calibration readings versus the post check readings. The RPD between the readings is calculated as
follows:
RPD = | R
x
- R
y
| x 100
0.5(R
x
- R
y
)
where R
x
= calibration reading
R
y
= post check reading
The precision of the laboratory analysis is assessed by the comparison of matrix spikes (MS) and matrix
spike duplicates (MSD), if required by the analytical method. The RPD between the analyte levels
measured in the MS sample and the MSD sample is calculated as follows:
RPD = | C
MS
– C
MSD
| x 100
0.5(C
MS
- C
MSD
)
where C
MS
= measured concentration of the matrix spike
C
MSD
= measured concentration of the matrix spike duplicate
For parameters where spiked samples are not practicable to assess laboratory precision, such as live
zooplankton, phytoplankton, and bacteria, a comparison of laboratory replicate analyses may be
performed in order to calculate the RPD.
For zooplankton samples collected at the MERC Mobile Test Platform, precision is measured by
analyzing at least two counting chambers from every sample collected. Precision is quantified by
calculating a coefficient of variation (CV) for each sample as follows:
CV = (S) x 100
(
x̄
)
For phytoplankton samples collected, at least two out of five treatment discharge samples and at least one
out of five control intake or discharge samples (from each set of five test trials) is selected for evaluation
of within-sample precision. Precision is measured by the analysis of at least two subsamples by the same
phytoplankton taxonomist. In the event that there are fewer than ten total control samples collected during
a treatment technology performance evaluation, a minimum of one discharge control sample is chosen for
evaluation of within-sample precision.