Exploratory analyses
To understand if the observed relationships were specific to
marijuana use or were also seen in cigarette use, we performed a
mixed-effects regression model in which frequency of cigarette use
(outcome), frequency of marijuana use, and symptoms of depression
were all time-varying, while gender and ethnicity/race were static.
Frequency of marijuana use and symptoms of depression were grand
mean centered. The random effects included intercepts (capturing
individual differences in the frequency of cigarette use at baseline)
and time trends (capturing individual differences in the rate of change
in frequency of cigarette use across the years of the study), with an
unstructured covariance structure.