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 [[variable]]s, 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]].
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A longitudinal study observes the same subjects, usually human beings, over a long period of time, usually to detect onset of disease.
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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.
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A longitudinal study differs in approach from a [[cross-sectional study]], which looks at something at a particular moment or snapshot in time.
A [[retrospective study]] is a less desirable type of longitudinal study that looks backwards rather than forwards in time.
A [[retrospective study]] is a less desirable type of longitudinal study that looks backwards rather than forwards in time.