What method involves separating individuals based on whether they have experienced a specific outcome at the beginning of the study?

The method referred to in the question is best characterized by case-control studies. In this approach, researchers identify participants based on their outcome status. Specifically, individuals are divided into two groups: those who have experienced the specific outcome (cases) and those who have not (controls). This method allows researchers to look back retrospectively to assess exposure to certain risk factors or interventions that may have contributed to the outcome.

Case-control studies are particularly valuable in situations where the outcome is rare or when the researcher is interested in understanding potential causes of the outcome after it has already occurred. By comparing the histories of both groups, researchers can identify associations between exposures and the outcome, which can offer insights into potential causal relationships.

In contrast, cohort studies follow groups over time to see if they develop the outcome based on their exposure status from the outset, while longitudinal studies track the same individuals throughout different time points to observe changes and outcomes. Descriptive studies provide a snapshot of characteristics in a population without establishing cause-and-effect relationships. Thus, the distinguishing feature of a case-control design is its focus on separating individuals based on their outcome status at the start of the research.