AbstractApproximately 125 carcinogenicity studies in Fischer 344 rats conducted by the National Toxicology Program (NTP) were examined to determine the frequency with which cage effects were associated with observed carcinogenic responses. All studies involving groups of 50 rats housed five per cage and showing evidence of chemically‐related carcinogenicity were considered. For each of these experiments, two statistical analyses were carried out for each dosed and control group: (i) a test to determine whether or not the occurrence of tumors clustered within cages; and (ii) an evaluation to determine whether or not tumor incidences differed significantly between differing cage shelf levels. These analyses showed that the numbers of statistically significant (P<0.05 orP<0.01) effects were consistent with the number expected by chance alone. Thus, cage‐related factors appeared to have little or no impact upon tumor incidence in these particular studies. Experimental design protocols now used by the NTP (which include random assignment of animals to cages; random assignment of columns of cages to dosed and control groups; and periodic rotation of cage location) further reduce the likelihood that factors associated with the housing of the animals could influence tumor incidence in current stud
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