Hi,
On 17 May 2007, at 17:25, Elizabeth Reynolds wrote:
> 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.
Indeed, that is correct - you need the second design.
The limitation of this correction is that it assumes that the
confounding effect of age is linear, but that may be ok.
> When I do this I get an error from FSL, though the analysis
> runs and produces viable looking results.
What is the error?
> 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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Stephen M. Smith, Professor of Biomedical Engineering
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