Using the Group-Based Dual Trajectory Model to Analyze Two Related Longitudinal Outcomes

The group-based latent trajectory model allows investigators to identify distinctive trajectory groups based on relevant outcomes. Dual trajectory analysis enables examination of the interrelationship between two outcomes simultaneously. This paper describes the application of the group-based dual trajectory model in mental health and substance abuse research. We analyzed two related recovery outcomes: 1) social contact with non-substance abusing friends and 2) stage of substance abuse treatment. We identified four groups for social contact and four groups for substance abuse treatment. We then examined the interrelationship between the two outcomes across the trajectory groups over 10 years. The two outcomes are positively associated longitudinally and evolve in the same direction, although for specific groups, the two outcomes do not covary. By examining dynamic linkages across all trajectory groups between two longitudinal outcomes, the dual model provides a more comprehensive and realistic understanding of the underlying relationships between the two outcomes under investigation.