Some cohorts in ENIGMA‐EEG have multiple recordings of their subjects in partial longitudinal study designs. Combined longitudinal/cross‐sectional designs allow investigation of age modulation of genetic risk. For example, the detrimental effect of the apolipoprotein E epsilon 4 (APOE4) allele and the protective effect of the epsilon 2 (APOE2) allele on the brain can be investigated using such age modulation models. Multiple observations per subject increases power and reduces confounding. However, the numbers do not quite reach those required to perform genome‐wide longitudinal genetic testing. Consistent with other genome‐wide studies, we therefore opted to start out by selecting a single observation per individual for the first runs of analyses. We hope and expect that with the increased availability of EEG data in people with genetic profiles, this will change in the future.
