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Hello -
Recently ran a DBM analysis on two groups (controls and patients) using the contrast 1 0; 0 1 with age or sex as the covariates of interest and uninteresting covariates. I have one set-up question: is it possible to run the analysis without specifying uninteresting groups weights and/or covariates?
Regarding the results (of which I have a bunch of questions), I understand that the mean difference display shows whether or not there is a significant difference in the mean vectors of the displacement of the deformations between the two groups (right?), but what do the arrows and the chosen coordinate indicate in the supplied image? Specifically, does the size of the arrow imply a larger displacement between the mean vectors of the two groups and does the coordinate given coincide with the region that had the largest difference between the two groups? Similarly, can one conclude that all of the vectors shown represent significant differences in shape or only those vectors with (and please forgive the ambiguity) 'substantial' length?
Questions regarding the graph: The x-axis lists the covariate of interest (e.g. age), but the range seems constrained form ~ -1 to .5 while the ages in our groups are much more distributed - and (shockingly) not negative - how should this be interpreted? Also the y-axis has a large range of values (raised to a power of 3) with no label... what does that range of numbers correlate with? The points in the graph that is generated are principally aligned into two vertical lines that parallel one another. Since I cannot seem to resolve the number of points that comprise either of the "lines", what do they represent (e.g. the number principal components, number of subjects, etc...)?
Finally (for now), how does one setup the canonical correlation? The menu asks for a selection to 'plot which vector' with 3 choices, followed by a choice of 'against which covariate of', also with 3 choices. This is confusing to me as I only had 2 covariates (age and sex) and I don't really know what the vectors in first option are. I am under the impression that this particular analysis will show which of the covariates had the greatest effect on the dependent variables (in this case the deformations), in contrast to the 'difference representation' which basically stated whether or not there was an overall difference. But I am not sure how to set it up to answer that question.
Thanks in advance...
Keith