Which best defines a longitudinal study?

A longitudinal study is best defined as one that examines changes in the same participants over time. This type of research design allows for the observation of how specific variables or outcomes evolve within the same group of individuals, providing insights into development, trends, or changes that occur due to various factors. By following the same subjects over extended periods, researchers can effectively assess causality and the dynamics of change, leading to a richer understanding of the phenomena being studied.

This approach contrasts sharply with cross-sectional studies, where data is collected at a single point in time, limiting the ability to analyze changes or causal relationships. While surveys can be used in longitudinal studies, not all longitudinal studies rely solely on survey methods. Additionally, the inclusion of multiple methodologies is not inherently characteristic of longitudinal studies, as the defining feature is the repeated observation of the same subjects over an extended time frame.