"Longitudinal studies are defined as studies in which the outcome variable is repeatedly measured; i.e. the outcome variable is measured in the same individual on several different occasions. In longitudinal studies the observations of one individual over time are not independent of each other, and therefore it is necessary to apply special statistical techniques, which take into account the fact that the repeated observations of each individual are correlated. The definition of longitudinal studies(used in this book) implicates that statistical techniques like survival analyses are beyond the scope of this book. Those techniques basically are not longitudinal data analysis techniques because (in general) the outcome variable is an irreversible endpoint and therefore strictly speaking is only measured at one occasion. After the occurrence of an event no more observations are carried out on that particular subject."
Excerpt from Applied Longitudinal Data Analysis for Epidemiology, A Practical Guide by Jos W. R. Twisk
