What defines a longitudinal study?

A longitudinal study is characterized by the collection of data from the same subjects repeatedly over an extended period. This methodology allows researchers to observe changes and developments over time, providing insights into trends, causality, and the dynamics of the subjects' behaviors or experiences. By tracking the same individuals, longitudinal studies can reveal patterns that might be missed in one-time data collection approaches, offering a deeper understanding of how and why changes occur.

The other options do not accurately represent the essence of a longitudinal study: a single data collection from diverse subjects reflects a cross-sectional approach rather than a longitudinal one, while focusing only on past events lacks the ongoing aspect integral to longitudinal research. Comparisons across different groups at a single moment also align with a cross-sectional design, further differentiating these methods from the longitudinal framework.