Hi
Thanks for below. There is another layer of complexity.
The covariates are scan-specific, and the covariate score exists for
the first scan only.
When prompted by SPM5 for the pair of covariate values, if one
enters the covariate value for scan 1 and a zero for scan 2, this is
inaccurate, as the model would use the difference (zero minus scan 1
covar value) as a change in score, which is not accurate, as
the scan 1 value is the score at baseline not the change over
time.
Is there a way to enter the covariate value at time 1 when no
value exists at time 2, in a manner that does not suggest that the
time 1 value is a change over time?
Thanks,
Steve
At 10:25 PM +0000 3/10/09, Jonathan Peelle wrote:
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