What is a cohort study primarily used for?

A cohort study is primarily designed to follow a group of individuals (the cohort) over a specified period to observe and measure outcomes related to certain exposures or interventions. This study design allows researchers to collect data over time, which can help establish temporal relationships between exposures and outcomes. By tracking changes and developments in health or behaviors within the cohort, the study can provide insights into cause-and-effect relationships.

Options that suggest analyzing participants at one point in time or focusing solely on retrospective data do not align with the primary purpose of a cohort study. Cohort studies are prospective in nature, meaning they focus on future outcomes, rather than being limited to data already collected. Additionally, comparing two unrelated groups does not capture the essence of a cohort study, which emphasizes monitoring the same group of individuals over time to assess changes and outcomes. Thus, the essence of a cohort study is its longitudinal approach, making option B the correct choice.