A longitudinal survey is a correlational research study that involves repeated observations of the same variables over long periods of time, often many decades. It is often a type of observational study, although they can also be structured as longitudinal randomized experiments. Longitudinal studies are often used in psychology, to study developmental trends across the life span, and in sociology, to study life events throughout lifetimes or generations. The reason for this is that unlike cross-sectional studies, in which different individuals with the same characteristics are compared, longitudinal studies track the same people and so the differences observed in those people are less likely to be the result of cultural differences across generations. Longitudinal studies thus make observing changes more accurate and are applied in various other fields.