Following your design matrix I would say that 0 0 1 -1 0 is the right
thing to do ie will tell you in which voxels slope sample 1 > slope
Usually I also run something like 0 0 1 0 0 and 0 0 0 1 0 (or using -1
in your case) allowing to have a more 'qualitative' interpretation, that
is sample 1 and sample 2 can both have a significant correlation but one
is stronger than the other or maybe only one sample shows the
correlation and that mean that in your patients there is no effect of
age which changes the way you can interpret the results
Hope this helps
> Dear SPM users,
> in my latest analysis I have two samples and I want to study the dependance of certain ROIs volume upon age.
> First I checked if the two samples come from two different populations: I prepared a full factorial considering one factor (=group), one covariate (age) interacting with the factor and another covariate (scolarity) not interacting. The matrix has therefore 5 columns: in the first two there are the images (sample 1 and sample2), in the third and the fourth there is the age of the two samples (age sample 1 and age sample 2) and in the fifth there is the scolarity of all subjects.
> In certain ROIs I found clusters in which there is a main effect of group and, in these clusters, volume turns out to be negatively correlated with age in both groups: the latter information comes from a separate multiple regression analysis.
> Now I want to check if the slopes of the regression lines are statistically different. How should I do? I have tried creating, for instance, a contrast with 0 0 1 -1 0, but I am not sure...
> Can anyone help me?
> Thanks in advance,
Dr Cyril Pernet,
SBIRC fMRI Manager
SFC Brain Imaging Research Center
Division of Clinical Neurosciences
University of Edinburgh
Western General Hospital
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