An experiment is designed to test whether a new drug can be used to reduce auditory hallucinations in schizophrenics.

An experiment is designed to test whether a new drug can be used to reduce auditory hallucinations in schizophrenics. Schizophrenics who experience auditory hallucinations are randomly assigned to receive either a placebo drug, a very low dosage, a moderate dosage, or a high dosage of the drug. The results of the experiment show that the moderate dosage is the most effective. What is the major problem with this experiment?