All laboratory rodents are not created equal. Even among highly controlled inbred strains, behavioral variability within the same group is a naturally occurring phenomenon. The following factors can contribute further to this variability and should be considered when selecting test subjects: Age, strain, sex, breeding environment, housing, pre-test experience, and environmental conditions.
Most behavioral tests require 8-12 characteristically matched subjects per group before statistically significant differences are detectable. This helps ensure that your outcome is a true representation of a behavioral phenotype and not an artifact of design. Certain protocols require that test subjects be trained at a particular task before performance is measured and occasionally some fail to “train to criterion” independent of treatment. When this occurs, subjects are excluded from the data. Whenever possible, begin with sample sizes of 12-15 per group to maintain optimal sample sizes for analyses.