The prominenceof the longitudinal study has grown tremendously in the past twenty years. The search for causal inference has led researchers in many fields to collect prospective longitudinal data in an effort to draw causal links between interventions and endpoints. In many cases, such studies involve rigorously controlled experiments-for example, prospective randomized single-center and multi-center clinical trials. In other cases, longitudinal data from naturalistic studies are either prospectively collected or retrospectively obtained to explore dynamically changing associations. For example, in a randomized clinical trial, investigators often collect prospective longitudinal data on one or more endpoints in response to a particular intervention relative to a control condition. The focus may be either to determine if there is a significant difference between control and treated individuals at the end of the study,