Number and Size of Groups

With the objective of an efficient approach to testing chemicals, there is no point in having more groups (or more animals per group) than are strictly necessary to attain the endpoint of reliable detection of toxic effects. 

Toxicity studies are undoubtedly expensive in both financial and resource terms. Part of the cost is related to the number of animals and the extent of clinical, necropsy, and histopathologic investigations required. Considering the inherent variability of biological systems, there must always be a balance between the number of animals theoretically required to detect all effects (from the weakest upward) and the number required to detect significant toxic effects. In a well-conducted study that goes according to plan, it is possible to use fewer animals. 

However, in tests of chemicals with unknown toxic characteristics, problems often arise because the actual responses of animals differ widely from those anticipated when the study was designed. To address this problem, it is prudent to increase the number of animals to ensure that animals are available at key points of a study to provide adequate information. In acute studies, the requirement for groups and number of animals in groups is related to the reliable determination of acute toxic effects and the estimation of a median lethal dose. In subchronic and chronic testing, the numbers are related to the detection of effects, providing sufficient animals for an acceptable investigation of toxic mechanisms and giving an indication of a no-effect level.  

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