A Factor Influencing the Significance of Radiation Mortality Experiments

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Abstract
The distribution of post-irradiation mortality among apparently identical cages of mice was studied. It was found that the mortality of any single cage may be strongly influenced by a cage effect, which disappears when the mice are “shuffled”, each to a different cage every 24 hours. As a result of cage effect, the mortality of any single cage or small number of cages is a significantly less reliable estimate of the mortality of the entire population that can be accounted for by random variation alone. Therefore, a substantial difference in mortality between two experimental groups, although statistically significant by conventional tests, may be spurious if the groups are housed separately. Possible causes of cage effect are discussed, but no investigation of the causes was made. Cage effect can probably be avoided by caging each mouse individually, and this was found not to alter radiation sequellae materially. It was shown that valid mortality comparisons can be made with multiple animal caging if each cage originally contains an equal number of animals from each of the groups whose mortalities are to be compared.

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