Tracking is the term sometimes used in the discussions of data from the longitudinal study, to describe the ability to predict the subsequent observations from previous values. Informally this implies that subjects who have, for instance, the largest values of the response variable at the beginning of the study tend to continue to have larger values. Formally a population is said to track with respect to the particular observable characteristic if, for each of the individual, the expected value of relevant deviation from the population mean remains unchanged over the time period.