Final answer:
A well-defined clinical study using a placebo involves comparing a new drug with a placebo in a double-blind experiment, ensuring that any observed effects can be attributed to the drug itself.
Step-by-step explanation:
Among the options provided, the one that exemplifies a well-defined clinical study using a placebo to evaluate the treatment group is a study comparing the effects of a new drug with a placebo on a group of patients. This type of study typically involves an experimental group receiving the medication and a control group receiving a placebo.
In order to ensure that the results are due to the drug itself, and not due to expectations or biases, such a study would ideally be a double-blind experiment, wherein neither the participants nor the researchers are aware of who is receiving the actual treatment and who is receiving the placebo. The use of a placebo in the control group effectively isolates the effect of the treatment by acting as a comparison against the effects of the placebo effect. This is crucial for establishing the effectiveness of a new treatment, as it helps to attribute the observed effects to the drug itself rather than to participant expectations or other psychological factors.