Which description best defines a cohort study?

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Cohort studies compare outcomes among groups of people who are alike and followed over time, usually based on whether they were exposed to a factor, and then measure how many in each group develop the outcome of interest. This forward-looking design establishes the temporal sequence between exposure and outcome and allows calculation of risk or incidence differences between groups. The key idea is comparing similar groups to see how the outcome unfolds differently, which helps reduce confounding and shows whether exposure is associated with the outcome.

The other descriptions describe different study designs: a randomized trial involves random assignment to exposure; a cross-sectional snapshot captures exposure and outcome at a single point in time; an ex post factor study (retrospective) starts with the outcome and looks back for exposure. The description that emphasizes comparing a specific outcome in groups of individuals who are alike best captures the essence of a cohort approach.