How is a cohort study structured?

A cohort study is structured around a group of individuals who share a common characteristic or experience within a defined timeframe. This group, or cohort, is followed over time to investigate the outcomes of specific exposures or interventions. The primary characteristic of a cohort study is that the subjects remain constant throughout the study period, allowing researchers to track changes and outcomes in the same individuals.

In this context, the statement about the subjects remaining constant while the cases chosen may vary accurately describes how a cohort study is designed. Researchers can analyze the data collected over time to identify trends, relationships, or causal links between the cohort's shared experiences and the outcomes observed.

The other options do not align with the fundamental design of a cohort study. Cohort studies do not typically involve random selection of subjects at each interval or the introduction of new subjects for each measurement. They maintain a consistent group to provide reliable longitudinal data. Hence, the correct understanding of the framework of a cohort study emphasizes the stability of the subjects involved over time, facilitating effective analysis of their outcomes.