Dear SPM experts,
Conducting 2nd-level factorial analysis, I found that a change in the order of
subjects (scans) specified in the design leads to small difference in
statistical results.
In more detail, I specified a 2x2 within-subjects design using
flexible factorial.
When I exchange the order of subjects
(e.g. using the "Subjects" option for specification of scans & factors, and
by "Replicating" a subject and then deleting the original),
results for a specific contrast are a little different.
I'm using SPM8 rev.5236.
So my questions are:
(1) Is this what is generally expected?
I think such effect of subject order is possible (e.g. when the modeled data are
not so uniform in scaling), considering that estimation procedure involves
some Gram-Schmidt-like matrix operations which would accumulate
rounding error differently depending on the order of modeled data.
But I'd like to make sure that the difference in results are not by some mistake
in the design specification.
(2) If this kind of specification order-effect is genuine, and when it
affects on
the significance of a result, what is a recommended way to address this?
Just sticking to the order of obtaining scans?
Best,
Taka
--
Takayuki Nozawa
Assistant Professor, Smart Ageing International Research Center
Institute of Development, Aging and Cancer (IDAC), Tohoku University
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