What is defined as a "longitudinal study"?

A longitudinal study is characterized by its approach to data collection over an extended period, focusing on the same group of subjects. This method allows researchers to observe changes and developments within that group, providing insights into patterns and long-term effects that can be attributed to time or interventions.

By consistently engaging with the same population, longitudinal studies can track variations in behavior, health outcomes, or other variables, leading to stronger evidence of causal relationships. This is critical in fields such as psychology, medicine, and social sciences, where understanding change over time is essential for drawing valid conclusions.

The other approaches mentioned involve different methodologies or scopes of study that do not emphasize the time dimension in the same way that longitudinal studies do. For instance, observing different subjects at one point in time captures a snapshot rather than change, while random sampling can introduce bias if the focus is not on a specific group over time. Surveys conducted among a broad population provide a wide range of data but lack the depth of individual longitudinal analysis. Thus, option C aptly defines a longitudinal study and its distinctive role in research.