Introduction

Longitudinal data are very common in practice, either in observational studies or in experimental studies. In a longitudinal study, individuals in the study are followed over a period of time and, for each individual, data are collected at multiple time points. For example, each individual’s blood pressure may be measured repeatedly over time, or each student takes multiple quizzes throughout a semester. See Section 1.2 for a more detailed discussion of longitudinal data and Section 1.3 for some real-data examples. Thus, the defining feature of a longitudinal study is that multiple or repeated measurements of the same variables (e.g., blood pressure, quiz score) are made for each individual in the study over a period of time.