Ratios of 0.25 to 4.00
Figure 9. Lognormal probability plot for soil TPH QC/QA
Figure 10. Frequency histogram for soil TPH QC1/QC2
concentration ratios between 0.25 and 4.00 for NPD results.
concentration ratios from NPD.
these ratios on lognormal paper (Fig. 9). How-
0.224.07, which is in good agreement with the
ever, the histogram of ratios for MRD results
arbitrary limits used to edit the data. The geo-
metric mean of the 82 QC/QA ratios between
0.254.00 form a rectangular distribution, thereby
precluding statistical analysis using a lognormal
0.841.15 and 99% tolerance intervals of 0.166.15.
model. The NPD results were probably more reli-
able than the MRD results because the former
tried to ensure that both QC and QA laboratories
used the same state-approved method. There is a
wide variation in both the extraction and analysis
procedures for TPH as a function of various state
requirements. We chose to analyze only the more
reliable NPD results; no further analysis was con-
ducted on the MRD results. This decision was
rooted in the conviction that future aspirations
should at least be based on the best of past per-
A histogram of the log QC1/QC2 results is
shown in Figure 10 and the lognormal probabil-
ity plot appears in Figure 11. Both plots provide
only fair fits to the lognormal model and there is
some uncertainty about the correct location of the
straight line in Figure 11. Nonetheless, the expec-
tation of lognormality and the absence of a better
model convinced us to proceed with this model.
The geometric mean of the 78 QC1/QC2 ratios
limits on this mean were 0.841.07 (Table B5). The
QC1/QC2 concentration ratios between 0.25 and 4.00
99% tolerance intervals for individual ratios were
for NPD results.