United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Las Vegas NV 89193-3478
Research and Development
EPA/600/S4-89/036 June
&EPA Project Summary
Direct/Delayed Response
Project: Intel-laboratory
Differences in the Northeastern
Soil Survey Data
D. D. Schmoyer, R. S. Turner, and D. A. Wolf
Soil samples collected during the
Direct/Delayed Response Project
(DDRP) northeastern soil survey were
analyzed using the procedures
described in the analytical methods
manual for the DORP by four different
laboratories. Inconsistencies in
results were reported by the different
laboratories regardless of use of
standardized procedures. Audit
samples Oa, A, Bs, Bw, and C
Horizons were included in this
survey. The audit sample data were
used for an interlaboratory
comparison to determine if there are
significant differences in the audit
sample values reported by the
laboratories This analysis compares
the laboratories to each other rather
than to a true audit value. Such
analysis has been completed for
each chemistry variable in the DORP
northeast soil chemistry database.
Various graphics display the results.
Numerous significant interlaboratory
differences were identified. The
impact of significant interlaboratory
value differences on data analyses is
difficult to assess. If the samples
from each sampling class, region,
and state were uniformly distributed
across all the laboratories, the
interlaboratory value differences
would likely have little effect on
analyses conclusions. However, this
is not the case. The significant value
differences among laboratories are
not consistent across all audit
horizons, which makes it even more
difficult to assess their effect on data
analyses. The averaging that takes
place during data aggregation will
likely negate the effect of
interlaboratory value differences.
However, it is not possible to test this
negation on a general scale.
This Project Summary was
developed by EPA's Environmental
Monitoring Systems Laboratory, Lag
Vegas, NV, to announce key findings
of the research project that is fully
documented in a separate report of
the same title (see Project Report
ordering information at back).
Introduction
Soil samples collected during the
Direct/Delayed Response Project (DDRP)
northeastern soil survey were analyzed
by four different laboratories. The
laboratories were required to use the
procedures described in the analytical
methods manual for the DDRP soil
survey. Despite the use of standardized
procedures, it is possible for laboratory
biases to cause consistent differences in
results reported by the different
laboratories.
Audit samples from Oa, A, Bs, Bw and
C horizons were included in the DDRP
northeastern soil survey. The audit
sample data were used for an
interlaboratory comparison to determine
if there are significant differences in the
audit sample values reported by the
laboratories. For each variable, the audit
sample results are presented graphically,
the significant interlaboratory differences
are displayed in tabular form, and
summary statistics of the DDRP routine
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sample results are provided to aid the
reader in determining the practical
importance of the observed differences.
Procedure
The method of analysis of variance was
used to determine significant differences
among the audit sample values reported
by the laboratories. The Scheffe multiple-
comparison procedure was used to
identify the specific laboratory
differences that were significant. All
significance tests were performed at the
0.05 significance level.
The statistically significant differences
among values reported by the
laboratories may or may not be of
practical importance to the data user.
These significant differences were
determined relative to the within-
laboratory variation observed in the audit
samples. If the variation in the audit
samples was smaller than the variation
observed in routine samples, the
interlaboratory differences may have little
impact on the conclusions derived from
the soils data. Therefore, the observed
interlaboratory differences were also
compared with the within-sampling-class
standard deviation estimates. When the
interlaboratory differences were large,
relative to this routine data variation, they
could affect inferences based on the
routine data and should be considered
when interpreting the results of data
analyses.
Results and Discussion
Numerous statistically significant
interlaboratory value differences were
found, however, only a few of these
differences are large with respect to the
variation observed in the routine data.
Interiaboratory value differences that
could cause problems in the analyses of
routine sample data occurred for iron in
0.002 M CaCI2, aluminum in citrate-
dithionite, aluminum in 0.002 M CaCI2,
aluminum in KCI, calcium in 0.002 M
CaCI2, cation-exchange capacity in
NH4CI, and very fine sand.
Conclusions and
Recommendations
The impact of the interlabora
differences on data analyses is diffici
assess. The soil survey design used
concept of sampling classes which gi
soil series that, in theory, should be
similar. If the samples from e
sampling class, region, and state v
uniformly distributed across all
laboratories, the interlaboratory v;
differences would likely have little el
on analyses conclusions. However, th
not the case.
Many of the proposed levels of <
evaluation require data aggregated to
sampling class level. This aggrega
involves some type of averaging wi
sampling class and horizon combinatii
This averaging will likely negate
effect of the interlaboratory VE
differences; however, this is very diffi
to test on a general scale.
researchers using the DDRP data she
be advised to include the appropr
caveats concerning interlaboratory v;
differences in the conclusions of tl
studies.
D. D. Schmoyer and D. A. Wolf are with Martin Marietta Energy Systems, Inc., Oak
Ridge, TN 37831-6285 and R. S. Turner is with Environmental Sciences
Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6285.
L J. Blume is the EPA Project Officer (see below).
The complete report, entitled "Direct/Delayed Response Project: Interlaboratory
Differences in the Northeastern Soil Survey Data," (Order No. PB90-219
4031 AS; Cost: $17.00, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Las Vegas, NV 89193-3478
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S4-89/036
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