Which type of nonexperimental research measures an outcome in groups of individuals who differ by a particular characteristic?

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In observational research, you compare outcomes between groups that differ on a specific exposure or characteristic without assigning that exposure yourself. The design that measures an outcome in groups split by a characteristic is a cohort study. It follows the exposed and unexposed groups over time (prospectively) or looks back at records (retrospectively) to compare incidence or risk of the outcome. This nonexperimental approach allows you to estimate how the characteristic relates to developing the outcome, without random assignment.

For context, case-control studies start with people who have or do not have the outcome and look back to see who was exposed; cross-sectional studies assess exposure and outcome at a single point in time; randomized trials involve random assignment to exposure, making them experimental rather than nonexperimental.