An experiment is to be conducted to compare the effect of digitalis on the contraction of the heart muscles of rats. The experiment is conducted by removing the heart from a live rat, slicing the heart into thin layers, and treating the layers with dosages of digitalis. The muscle contraction is then measured. If four dosages, A, B, C, and D, are to be employed, what advantage might be derived by applying A, B, C, and D to a slice of tissue from the heart of each rat? What principle of design is illustrated by this example?