Hi there,
I am running an analysis to examine differences in cortical thickness between two groups (clinical vs control) and have entered ICV as a nuisance variable and age as a covariate. The analysis results in qdec do not appear to give a result that answers the question: does the average cortical thickness differ between clinical and control with both the nuisance factor and the covariate accounted for. The only result comparing average thickness between the groups lists ICV as a nuisance factor, and does not include age as a covariate. Other results are examining correlation between age and thickness accounting for group, or whether the thickness-age correlation differs between groups accounting for ICV.
A second question: when i just include ICV as a nuisance factor and do not enter age as a covariate (groups mean age is almost identical), there are some areas of cortical thickening in the clinical group, which become much larger after correction for multiple comparisons using Monte Carlo Null-Z simulation (threshold 0.05). This seems counterintuitave to me as I would expect a reduction in significant clusters after correction. One of the main clusters traverses the cingulate gyrus, however, when I extract cortical thickness values for each individual subject and compare group means for subregions of the cingulate in SPSS, there are no significant differences on any of the subregions. In fact, they are almost identical. Could adding ICV as a nuisance variable account for this? Mean ICV values are almost identitical between groups. Or is there some additive affect at the cluster level?
Many thanks for any clarity you can provide.
Kind regards,
James