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NIST Results
Determination of Assigned
Values
Determination of assigned values: For a particular analyte,
the performance on the reference material was deemed acceptable
for the purpose of this exercise if the laboratory result
was within 30% of the confidence interval for analytes listed
in the Certificates of Analysis for Certified/Standard Reference
Materials. For each analyte of interest not certified in
these materials, a "target" concentration and
the associated uncertainty were calculated.
Laboratory results within target upper and lower limits,
typically 30 to 40%, of these concentrations were deemed
acceptable for this exercise. If a laboratory demonstrated
acceptable performance on a particular analyte in the reference
material, that laboratory's results for that analyte in
the corresponding "unknown" exercise material
was then used in the calculation of the analyte's exercise
assigned value unless it was deemed an "outlier."
For evaluation of potential outliers, statistical tests
and expert analyst judgement were used after viewing both
normal and log plots of the data. This judgement utilized
knowledge of potential co-eluters based on the laboratory's
reported methods. In instances in which the analyte concentration
was below the detection limit of most participating laboratories,
no exercise assigned value was calculated. In data sets
such as this with a number of laboratories reporting results
as "not detected" at various detection limits,
there is no consensus as to what "numerical" value
should be assigned to these results in the computation of
grand means, etc., e.g., "0," ½ Detection
Limit (DL), and the DL value itself have all been used and
the choice is influenced by the use of the particular data
set.
Determination of laboratory analyte means: The laboratory
analyte mean of the replicate (S1, S2, and S3) results was
calculated. Non-numerical data were treated as follows:
A mean "<value" was used when three "<values"
were reported; NA (not analyzed/determined) was used for
three reported NAs, etc.; and, if the reported results were
of mixed type, e.g., S1 and S2 were numerical values and
S3 was reported as "<value", the two similar
"types" were used to either determine the mean
or to set a non-numerical descriptor.
