On Thu, 17 Feb 2005 11:29:19 -0000, Alexa Morcom <[log in to unmask]> wrote:
>Steve/ list -
Thanks for replying to my post.
>What exactly seems to have gone wrong with the nonsphericity
specification?
>
>As I understand it, subject/ replication *is* a factor, it's just the
random
>factor, and thus the one that covariance components for the group/s are
>estimated over.
Right. I meant that "subject" is a random factor, but that it wasn't
right to consider the different subjects as different levels of this
factor. (Rather, that there would be only one level, which
was "random.") Looking at some stats texts, I was wrong.
>Therefore I believe you should have xVi.Vi (later SPM.xVi.Vi) = {[25x25
>double] [25x25 double]}, where: xVi.Vi{1} is [eye(15) zeros(15,10);
>zeros(10,25)] representing the covariance component for variance of the
>first group, and xVi.Vi(2) is [zeros(15,25); zeros(10,15) eye(10)]),
>representing the equivalent for the second group.
That's what I got if I used answered "group (2)" to "replications are
over?" and "no" to "correlated repeated measures?"
I take it that's what you think I should choose.
>But I can't see where subject 1 of group 1 is identified with subject 1 of
>group 2
With any other choices, there is such an identification. This is because
of xVi.I, where the subjects for group 1 are levels 1:15 and the subjects
for group 2 are levels 16:25. But maybe this is irrelevant insofar as the
(proper?) choices listed above make the code behave correctly.
I probably should have re-read the (extensive) "replications are over"
discussions on the list and made the choices properly first. But I looked
in SPM.mat and saw the subjects labeled as I mentioned in the previous
paragraph, and that confused me.
Thanks,
S
>
>Any help?
>
>Alexa
>
>
>
>| -----Original Message-----
>| From: SPM (Statistical Parametric Mapping) [mailto:[log in to unmask]]On
>| Behalf Of Stephen J. Fromm
>| Sent: 16 February 2005 14:24
>| To: [log in to unmask]
>| Subject: [SPM] Non-sphericity, PET "Compare-populations" model, and
>| factor indices
>|
>|
>| I'm trying to use the PET "Compare-populations: 1 scan/subject (two
sample
>| t-test)" model.
>|
>| The confusing part is when I get to the non-sphericity questions.
>|
>| It's also not clear to me that spm_spm_ui('Files&Indices',...) is
>| correctly computing the factor indices.
>|
>| I have two groups, of size 15 and 10. This produces a 25x4 factor
>| matrix, "I". The last two columns are all ones. The first column,
>| representing "subject" according to D.sF, is of the form
>| [1:15, 1:10]'
>| The second column, representing "group," is
>| [ones(15,1); 2*ones(10,1)]
>| Subsequent code in spm_spm_ui seems to interpret the first column
>| literally as factor levels for factor "subject." But that doesn't seem
>| right. First, I don't see why "subject" is a factor with distinct
levels
>| in a two-sample t-test (in the context of subjects drawn from two
>| populations, with inferences to be drawn at the population level).
>| Second, the code seems to identify subject 1 of group 1 with subject 1
of
>| group 2, and on up to subject 10 of group 1 and 2, when in fact these
are
>| distinct subjects. This then leads to what appears to be incorrect
>| specification in the non-sphericity module. (Since the subjects are
>| distinct, there are no repeated measures, but of course the two groups
can
>| have different variances.)
>|
|