I'd like to do a group analysis where the covariate of interest is a behavioral measure collected outside the scanner from a task in which two participants were paired (matched on age and gender). For some dyads, both members will be part of the fMRI analysis, whereas for others only one member has fMRI data. The effect of interest is individual (i.e., does this behavioral measure relate to brain activation in the same individual), but I'm wondering if I should try to account for possible non-independence in the data using a LME model in AFNI. This scenario doesn't seem to fit the typical suggested use of 3dLME (i.e., within-subject repeated measures). Also, is this advisable when only part of the sample represents complete dyads, or would it be better in this case to treat everyone as individual and just use, e.g., 3dMEMA? From my understanding, ignoring non-independence will not bias effect estimates, just the standard error, and I was already planning to use a liberal significance threshold, as this is an exploratory analysis.

If 3dLME is recommended, could you provide an example of the syntax for a dyadic analysis with covariates at both level 1 (between individual subjects) and level 2 (between dyads) and using a compound symmetry error structure?

Finally, could you clarify whether this warning might apply to my situation: "Between-subjects covariates are generally acceptable. However EXTREME caution should be taken when the groups differ significantly in the average value of the covariate." Would "groups" in this case refer to different dyads?

Many thanks in advance!