In an experiment, the control group is defined as:

• A. The group that receives the treatment
• B. The group where the outcome is measured after treatment
• C. The group that is not exposed to the treatment; serves as a comparison
• D. A group that is used to calibrate instruments

Answer: (C)

In experimental design, a control group provides a baseline for comparison. It does not receive the experimental treatment, so any difference in outcomes between groups can be attributed to the treatment itself rather than other factors. This setup helps account for placebo effects, natural variation over time, and measurement variability. A related idea is often using a placebo to keep participants blind to group assignment, but the essential point is that the control group is not exposed to the treatment and is used to compare against the treated group. The other descriptions—one that receives the treatment, one that focuses on when outcomes are measured, and one about instrument calibration—do not define the control group in the experimental sense.