Distinguish between a randomized controlled trial and a cohort study.

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The key idea is the difference between intervention with random assignment and observational grouping. In a randomized controlled trial, participants are randomly assigned to receive the intervention or a control, and outcomes are tracked going forward. This randomization helps balance both known and unknown confounders across groups, making it possible to draw stronger causal inferences about the intervention’s effect. In a cohort study, researchers observe people based on their exposure status and follow them over time to see outcomes, but there is no random assignment to exposure, so differences between groups can reflect other factors and analyses focus on associations rather than proof of causation.

The other statements don’t fit because they invert or misstate the main features: an RCT is not merely observational and a cohort is not interventional with random assignment; the temporal data aspect (prospective vs retrospective) can vary for both designs, but the defining distinction is randomization and intervention versus observation. Additionally, randomized trials are designed to control for confounding through randomization, whereas cohort studies do not have that built-in control, so they do not achieve perfect randomization.