The longitudinal study of the normalized myelin content of the regions with significant age-related changes analyzed the normalized myelin content of the 185 NCANDA adolescents that had longitudinal data from baseline to Year 2 (see Table 1). For each of these regions, the age effects were estimated by fitting a linear mixed effects model (Bemal-Rusiel et al. 2013) with the fixed effects being age (age at baseline), age2, age*sex, sex, time (time passed since baseline visit), ethnicity, svol, and site, and the random effects being time for each subject s and time point t myelins,t∼β0+βage⋅ages+βage2⋅ages2+βage∗sex⋅ages∗sexs+βsex⋅sexs+βtime⋅times,t+βethnicity⋅ethnicitys+βsvol⋅svols+βsite⋅sites+b0s+b1s⋅times,t.
