Dr. Kong studies the neural basis of learning and memory, which is often related to the hippocampus in the medial temporal lobe. To test a hypothesis about the role of the hippocampus in learning, Dr. Kong gave excitotoxic lesions to the hippocampus of a group of mice and compared their memory abilities to those of mice without brain damage. Once the lesion group recovered, all mice performed the same learning task. Dr. Kong found that the brain-damaged mice showed less learning. What is missing from Dr. Kong's research design?