Hello all,
I am comparing two groups performing a cognitive task. The two groups
differ significantly in age but this is unintentional so I want to use
age as a covariate to ask the statistical question, "if these two
groups were the same age, what activation pattern would I see?". I
have seen posts suggesting that I run this analysis in FSL with age as
a separate EV for each group where the ages are demeaned from each
groups mean separately. See below. This model is simplified as I actually
have 9 subjects in each group.
Group EV1(group1) EV2 (group2) EV3 (age group1) EV4 (age group2)
1 1 0 4 0
1 1 0 -2 0
1 1 0 -3.5 0
1 1 0 1 0
2 0 1 0 -.75
2 0 1 0 3
2 0 1 0 -2
2 0 1 0 2
My concern with this method of analysis is that it is not accounting
for differences that may exist between my groups. Instead I think
that I should to set up my analysis as below with one EV for age,
where ages are demeaned from the grand mean age of the two groups
combined. When I do this I get an error from FSL, though the analysis
runs and produces viable looking results. Is the method below the
proper way to set up an analysis to ask the question, "if these two
groups did not differ by the covariate, what would the activation look
like?" and if so is it safe to trust the results FSL is giving me
despite the error message? I believe I could get rid of the error message
by modeling my analysis as if all subjects came from the same group (having
all 1s in the left hand column), however, this does not seem appropriate
especially since one group has a genetic disorder and the other does not.
Please let me know what you think.Thanks.
Group EV1(group1) EV2 (group2) EV3 (age )
1 1 0 3
1 1 0 1
1 1 0 -2.5
1 1 0 4
2 0 1 -1
2 0 1 2
2 0 1 1.7
2 0 1 1.5
Liz Reynolds
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