Longitudinal data is characterized by repeated observations on each individual u nder study.
This allows inferences to be made about the change in a response thr ough time. The strength of a longitudinal analysis is that it allows us to accur ately identify the effect of a change in one variable on another by separating c ohort and age affects. Such a separation permits a greater casual interpretation of an association between two variables. In a cross sectional study, where meas urements are taken on individuals only once, an observed association between var iables X and Y may be confounded with differences amongst individuals, or so-cal led cohort effects. When this occurs the association between X and Y does not ne cessarily apply to an individual.