Longitudinal study

A longitudinal study is a study that involves the repeated observation or examination of the same set of subjects over time with respect to one or more study variables, particularly in medical research where the subjects are living beings. A longitudinal study is often desired because it follows changes in the same people and can be helpful to detect the onset of disease.

In contrast, a cross-sectional study compares results across different people at a particular moment or snapshot in time, which can introduce undesired variables to distort the results.

A retrospective study is a less desirable type of longitudinal study that looks backwards rather than forwards in time.