Consider an experiment being designed to study the effectiveness of an experimental pain reduction medication. The plan includes recruiting 100 individuals suffering from moderate to severe pain to participate. One half of the group will be assigned to take the actual experimental drug, and the other half will be assigned a placebo. The study will be blind in the sense that the individuals will not know which treatment they are receiving. At the end of the study, individuals will be asked to record using a standardized scale how much pain relief they experienced. Why is it important to use a placebo in such a study?