What research design is characterized by observing changes over time in a single group?

The correct response is characterized by tracking a single group over an extended period to observe changes and developments in their characteristics, behaviors, or outcomes. This design is particularly effective in understanding trends and the effects of time on the specific group in question. For instance, researchers might use this method to study how a particular intervention impacts health outcomes by consistently measuring the same participants at various time points.

In contrast, cross-sectional designs capture data at a single point in time, cohort designs often involve comparing multiple groups across time rather than focusing on a single group, and case-control studies look retrospectively at individuals with a specific outcome and compare them to those without it, rather than tracking changes over time. Each of these alternative designs serves different research purposes and benefits but does not emphasize the continuous observation of a single group as effectively as the longitudinal design does.