Modeling Intensive Longitudinal Study Using a Mixed-Effects Location Scale Model: Examining the Mean and Within-Subject Variance of Time to First Cigarette

• Activity Number: 475
• Type: Topic Contributed
• Date/Time: Wednesday, August 12, 2015 : 8:30 AM to 10:20 AM
• Sponsor: ENAR
• Author(s): Donald Hedeker* and Robin Mermelstein
• Companies: The University of Chicago and University of Illinois at Chicago
• Keywords: variance modeling ; heterogeneity ; random scale ; EMA data

Abstract

Intensive longitudinal data are increasingly obtained in health studies to examine subjective experiences within changing environmental contexts. In this presentation, we focus on data from an adolescent smoking study using ecological momentary assessment in which there was interest in examining the time to first cigarette in a day, a reported marker of smoking dependency. Specically, we examine whether this outcome was related to an adolescent's smoking level, both in terms of the mean and the within-subject variance using a mixed-effects location scale model as implemented in the software program MIXREGLS. The mixed-effects location scale model simultaneously models the mean, between-subject (BS) and within-subject (WS) variance, and includes random subject effects for both the mean (location) and within-subject variance (scale). The analyses indicated that subjects with increased smoking level smoke, on average, earlier in the day, and also were more consistent (lower WS variance) in their time of first cigarette.