Steve/ list -
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.
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.
But I can't see where subject 1 of group 1 is identified with subject 1 of
group 2
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.)
|
|