How spacing of data collection may impact estimates of substance use trajectories.

The goal of this study is to provide an empirical example using longitudinal cigarette smoking data that compares results of growth mixture trajectory models on the basis of contiguous and snapshot measurements. Data were drawn from an intensive longitudinal study of college freshman (N = 905) with a previous history of smoking. Participants provided weekly smoking reports for 35 consecutive weeks. We found that using contiguous weekly data (35 waves) or 6-wave or 4-wave snapshot data provided similar trajectory curves and proportions. However, there were notable differences in individual trajectory assignments on the basis of contiguous and snapshot measurements.