Badreddine,
>
> Dear SPM ers:
> I recently used SPM96 to compare 10 subjects, 2 conditions (bas vs act)
> using multisubject different conditions design.
> contrast 1 -1 Bas > Act showed one cluster in the frontal cortex at p <
> 0.005, size = 64 voxels.
> I plotted the data and I got the following values
>
> at voxel -56, 10, 28 for
> bas 128.87 +/- 2.4684
> act 114.56 +/- 2.4684
>
> and for another voxel at -52, 10, 26
> bas 161.09 +/- 4.9552
> act 149.65 +/- 4.9552.
>
> Interestingly plot of act along the y axis ( . . . . . . ) is
> identical but inverted compared to bas ( . . . . . . ) and shifted
> down.
>
> Obviously variance of bas versus act is the same for same voxel location
> and is heterogeneous between voxels.
> Can someone explain why the variance between bas and act for a given voxel
> location
> is absolutely identical ? Is it a constraint inherent to the math of SPM
> or possibly a simple statistical relationship that I disregarded?
It is actually a combination of both. The constraint (or more correctly, the
assumption) inherent in the modeling implemented by SPM is that of
identically distributed errors. This implies that the time series errors
are the same during each condition. In addition to the properties of the
time series errors, the variance of a condition effect also depends on
the design matrix. It can be induced that there were an equal number
of observations for bas and act, as in that case the design matrix
factor would be equal for bas and act. If, for argument's sake, the number
of observations associated with bas was greater than that associated
with act, then you would see that the variance of the bas parameter estimate
was smaller than that associated with act.
Sincerely,
Eric
Eric Zarahn
University of Pennsylvania
> Thanks for your input.
>
>
>
>
>
> Badreddine Bencherif, MD
> Division of Nuclear Medicine
> Johns Hopkins University School of Medicine
> 601 N. Caroline St. / JHOC 4230
> Baltimore, MD 21287-0855
>
> Phone: (410) 614-2787
> Pager in hospital: 3-2050 out hospital: (410) 283-2050
> Fax: (410) 614-1977
> email: [log in to unmask]
>
>
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