Longitudinal data refers to information collected from the same subjects repeatedly over a period of time. This type of data allows researchers to observe changes and trends within individuals or groups, making it valuable for studying development, behavior, and outcomes. For example, researchers might track the health of a group of patients over several years to understand the long-term effects of a treatment. This data collection method contrasts with cross-sectional data, which captures a snapshot at a single point in time. Longitudinal studies can provide insights into cause-and-effect relationships, as they help identify patterns and shifts that occur over time, enhancing the understanding of various phenomena.