Which of the following best describes the primary subjects of a longitudinal study?

The correct description of the primary subjects of a longitudinal study is that the same subjects are studied multiple times over a period. This method allows researchers to observe changes and developments in the same individuals, providing insight into how certain variables may influence behaviors or outcomes over time. By following the same subjects, researchers can identify trends, causations, and long-term effects that may not be apparent in a cross-sectional study, where different subjects are observed at a single point in time.

Longitudinal studies are particularly valuable in fields such as psychology, sociology, and medicine, where understanding progression and the impact of time on certain factors is crucial. This repeated measurement enables researchers to collect data that reflect temporal sequence and can enhance the understanding of cause-and-effect relationships.

Other options do not accurately capture the essence of a longitudinal study, as they involve varying subjects or do not focus on the repeated measurements of the same individuals.