In epidemiology, a longitudinal study refers to the collection of data from the same unit (e.g., the same person) at two or more different points in time. The great advantage of longitudinal studies is that each subject serves as his or her own control in the study of change across time. This reduces between-subject variability and requires a smaller number of subjects compared with independent subject designs, and it allows the researcher to eliminate a number of competing explanations for effects observed—most important, the cohort effect. The main disadvantages of longitudinal designs are that they are expensive and time-consuming relative to cross-sectional designs, and that they are subject to difficulties with retention, that is, subjects may drop out of the study. In addition, special statistical ...

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