Which research method uses the same subjects throughout the entire study?

The correct answer is a longitudinal study because this research method involves collecting data from the same subjects multiple times over an extended period. This allows researchers to track changes and developments within the same individuals, providing valuable insights into trends and long-term effects.

Longitudinal studies are particularly effective for understanding developmental processes or the progression of certain conditions or behaviors over time. By analyzing data from the same subjects at different points, researchers can draw more robust conclusions about causal relationships and changes that occur with time, which wouldn't be possible with studies that involve different groups at varying points in time.

Other research methods mentioned, such as cross-sectional studies, involve examining different subjects at a single point in time, which limits the ability to observe changes in the same individuals. Observational designs can also vary in terms of subject selection and may not focus on the same individuals over time. Repeated measures design may involve the same subjects across different conditions, but it typically refers to a specific experimental context rather than tracking the same subjects over a prolonged period like a longitudinal study does.