Hi Steve
Are your covariates subject-specific, or scan specific? I.e. do you
enter the same number for the subject on both scan 1 and scan 2? With
a paired samples t-test, I think any part of your data that can be
explained in a subject-specific manner would be modeled out; so, if
your covariates are subject-specific, they aren't helping you explain
any more of your data. This would explain why your results are the
same.
If your covariates aren't set up this way, then something else is
likely to be the culprit...
Hope this helps,
Jonathan
On Tue, Mar 10, 2009 at 8:22 PM, Steve Cramer <[log in to unmask]> wrote:
> I have scanned 24 subjects with fMRI twice per person. I am trying to
> examine a paired t-test while controlling for effects of two covariates. I
> am able to generate a paired t-test in SPM5 without the covariates, and the
> activation looks proper.
>
> However, the results (glass brain, cluster analysis) do not change when I
> add 1 or 2 covariates to the model. When I made the second model (paired
> t-test that has the two covariates), I used all the same choices (same
> pairs, no change in Grand mean scaling choices, etc) for the paired t-test,
> and note too that the covariates are entered without error (vector entered
> ok, variable named OK, no interactions, etc). In the second model (with 2
> covariates), I entered a zero for both of these covariates in the contrast
> manager.
>
> Thus, it appears that in SPM5, a paired t-test with no covariates produces
> identical results as a paired t-test with 2 covariates properly specified.
>
> I would expect that the second model, with the two covariates that have a
> zero in contrast manager, would have a different result than the first
> model, with the difference reflecting removal of the signal accounted for by
> these two covariates. What am I missing, or doing wrong ? Thanks--Steve
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