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

Abstract

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.

Fig. 1

Plots of BIC’s for different models using contiguous (upper plot), 6-wave (middle plot), 4-wave data (lower plot)

Fig. 2

Trajectories of weekly average smoking quantity (contiguous data, 5-group cubic)

Fig. 3

Trajectories of weekly average smoking quantity (6-wave data, 5-group, cubic)

Fig. 4

Trajectories of weekly average smoking quantity (4-wave data, 5-group, cubic)

Figure 5

Typical Case 1: Observed versus contiguous estimated trajectories for a subject classified in group 1(non-smoker) based on the contiguous data model and in group 2(early quiter) based on 6-wave data. Note: The observed contiguous trajectory is represented by small black dots and the observed 6-wave snapshot trajectory is represented by the encircled black dots. The estimated trajectories for groups 1(non-smoker) and 2(early quitter) based on contiguous data or 6-wave data are represented by the numbers 1 and 2, respectively.

Figure 6

Typical case 2: Observed versus contiguous estimated trajectories for a subject classified in group 1(non-smoker) based on the contiguous data model and in group 2(early quitter) based on 6-wave data model. Note: The observed contiguous trajectory is represented by small black dots and the observed 6-wave snapshot trajectory is represented by the encircled black dots. The estimated trajectories for groups 1(non-smoker) and 2(early quitter) based on contiguous data or 6-wave data are represented by the numbers 1 and 2, respectively.