Hi Gang,
Thank you for the reply! I changed the random-effect specification to "~1" and it worked this time. However, I am not sure if this is the same model I was planing to use for this data. Does "~1" only model a random intercept or model a random slope as well? Essentially I want something like ( 1 + ScanPoint | Subject) + ( 1 + ScanPoint | Scanner), so each subject and scanner have a random slope and a random intercept.
Regarding Scanner, this is a bit tricky here since Scanner is both a within-subject variable and between-subject variable in this dataset. This is multi-site longitudinal data so each site has its own scanner(s). For some sites more than one scanner was used (in which case different ScanPoints were scanned with different Scanners). Now my model looks something like
-model "Group*ScanPoint+Sex+Age+TIV+Scanner" \
-ranEff "~1" \
Do you think this is the right model to use?
Thank you!
Best,
Hengyi