Which statement best differentiates observational studies from experimental studies?

• A. Observational: observe events without manipulation; Experimental: manipulation by researcher which individual receives exposure or not.
• B. Observational: involve randomization of participants; Experimental: observe outcomes without intervention.
• C. Observational: require a control group; Experimental: do not use control groups.
• D. Observational: cannot estimate risk; Experimental: can only estimate risk.

Answer: (A)

The essential distinction is who controls the exposure. In observational studies, you watch what happens in the real world and do not assign or alter who is exposed to the factor of interest. In experimental studies, the researcher actively manipulates the exposure and decides which individuals receive it.

That’s why the best statement is the one that says observational studies observe events without manipulation, while experimental studies involve manipulation by the researcher of who receives exposure.

Other choices misstate the basics: randomization is a feature of experimental designs, not observational; requiring a control group is not exclusive to observational studies, and experiments typically include a control group; and both observational and experimental designs can be used to estimate risk, so risk estimation isn’t what fundamentally differentiates them.