Refinement and reduction through the control of variation.

The key to doing animal experiments efficiently, while using the minimum number of animals without loss of scientific information, lies in good control of random variation, and recognition and control of "fixed effect" variation, such as the sex or strain of the animals. However, many scientists erroneously assume that the use of outbred, genetically heterogeneous animals is justified, because in some way, they more closely model humans. Unfortunately, all this does is to increase the phenotypic variation, which results in less-powerful experiments. If the aim is to model variation in human responses, this can be done by using a small number of animals from several isogenic strains, without increasing the total number of animals. Reducing inter-individual variation, whether caused by genetic or non-genetic causes, will nearly always result in improved experiments. Fixed-effect variation, such as the sex of the animals, can be taken into account, either by restricting the conclusions to the sex actually used, or by assuming that the other sex would respond in the same way, or by including both sexes in the study, by using a factorial design, without increasing the total number of animals.