In which type of observational study are outcomes followed over time?

The correct answer is prospective studies, as they are specifically designed to follow participants over a period of time to observe outcomes as they occur. In this type of study, a group of individuals is recruited, and data is collected at multiple points in time, allowing researchers to assess how certain exposures or interventions may impact health outcomes.

Prospective studies can provide valuable insights into the temporal relationship between exposures and outcomes, thereby facilitating a better understanding of causal links. For instance, if researchers want to investigate how lifestyle factors influence the development of a certain disease, they would track a cohort of individuals over time, recording relevant data, and then analyze the outcomes as they develop.

In contrast, cross-sectional studies capture data at a single point in time and therefore can't show how outcomes develop over time. Retrospective studies look backward in time to examine exposures and outcomes that have already occurred, relying on historical data. Case studies focus on detailed reports of a single individual or a small group, often lacking the broader statistical analysis that prospective studies utilize. Thus, prospective studies are unique in their forward-looking approach, making them the appropriate choice for observing outcomes over time.