Answer:
independent variable control
Step-by-step explanation:
Placebos have no direct impact on the variable being studied. They are used to protect the validity of the test results. If both a effective drug and a placebo were used in a trial, the expectation is that those on the drug will improve, but those on a placebo will not exhibit any improvements beyond what is normal, or expected. If subjects from both trisl exhibited the same level of improvement, then there exists a third variable that must be considered. Fior example, the individuals on the placebo may think positvely if they thought they were getting the real drug. That effect should be the same for the group receiving the actual drug. Or perhaps environmental conditions changed that impacted both groups equally.