Joint International Conference on Long-term Experiments ...

these general patterns appear to be identical between treatment groups, then that would be
evidence against this hypothesis.
SIX AZO DYES
As part of its ongoing program applying a “carcinogenesis” bioassay to chemical
compounds now in use, the NTP has reported on studies involving six AZO dyes. These
assays were conducted at a single site. Each dye was subjected to the same protocol, in
which male and female rats and mice were divided into three groups each: controls, low
dose, and high dose. There was an exception to the standard protocol. For three of the dyes,
single control groups of 90 animals each were used in the male and female rat portions of
the assay. Otherwise, there were separate controls for each sex/species/compound of 50
animals each).
This provides us with a repeated experiment against which to test the hypothesis
proposed above, using the method described in subsection A in the preceding section.
Three of the six dyes were declared “carcinogenic” for at least one speciesor sex. The
marginal counts of animal with tumors in specific sites (the usual NTP analysis) suggested
dose-related changes for all but 3 of the 48 sex X species comparisons. As indicated above,
however, multiple testing for tumors in specific sites raises the question of how to adjust the
nominal p values to keep the false-positive rate from going too high. Other suggestions have
been made based on the use of Bonferonni bounds and the statistical characteristics of the
tests used. Most of this amount to requiring a lower level of significance (usually around
0.01) for frequently occurring tumors. When this is done, all six dyes have significant doserelated
shifts in tumors for at least one sex or species and 36 of the 48 sex X species
combinations still show significant dose-related changes.
Analyses like these have been criticized because they are based upon multiple testing
and because the p-value adjustments tend to be of an ad hoc nature. To get around such a
criticism, I ran overall tests of shifts in tumor patterns as described in the preceding section
for all six studies.
Using methods described in the Appendix, data for each animal were converted to a
vector of 25 numbers, representing the maximum degree of dysplasia discovered in specific
organ systems, along with the number of weeks the animal was on study. Organized this
way, the 150 animals in a given sex X species X compound study can be thought of as a
cloud of points in multidimensional space. Animals with similar patterns of neoplastic
lesions will tend to provide points close together.
If there were no effect of treatment on neoplastic lesions, then the cloud of points
would be such that animals in a given treatment group would be randomly scattered thought
the space. The points closest to a given animal’s point would belong to animals associated
with any of the three treatment groups with equal probability. However, if treatment had an
effect on neoplastic lesions, we would expect animals in a given treatment group to be
clustered together more so than might be expected at random.
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