What is the purpose of a control group in epidemiologic studies?

• A. To randomize participants to exposure groups.
• B. To provide a baseline for comparison to estimate the effect of exposure.
• C. To ensure all participants receive the intervention.
• D. To collect data only on the outcome of interest.

Answer: (B)

The main idea is using a baseline for comparison to judge what the exposure does. A control group provides a reference point that represents what happens in the absence (or different level) of the exposure, so differences in outcomes between groups can be attributed, at least in part, to the exposure itself rather than other factors. This comparison helps estimate the effect of the exposure by showing how risk or outcome changes when exposure is present versus not present, while accounting for background disease risk and potential confounders when possible.

In practice, the control group is the unexposed or standard-treatment group that mirrors the exposed group in all respects except for the exposure being studied. Randomization, when used, helps ensure the groups are comparable, but the fundamental purpose remains to provide a baseline for estimating the exposure’s effect. The other statements describe design features or conditions that don’t define the control group’s primary role: randomizing participants is a method used in some studies, not the sole purpose of a control group; making sure everyone gets the intervention would remove a comparison group; and collecting data only on the outcome overlooks the need to compare against an appropriate baseline.