Dear SPM ers:
When reporting SPM99 results for PET scan
obviously T or Z of all regions above significant threshold with
x,y,z coordinates (MNI transformed to T & T etc ... should be
clearly stated).
I obtained also for a given height threshold of p of 0.001 and a size
threshold of k=62 for two different experiments ( first in a large
group 42 and second in a smaller SUBSET of 15 subjects of the large
group) the following
1) T and p values for size
n=42, df=37 , height T =3.33 and p value for k=0.038
n=15, df=12, height T=3.93 and p value for k=0.023
I understand (at least I think I understand) why there are
differences (EC of different T fields with different d.f.)
2) smoothness FWHM
n=42: 11.5, 13.2, 13.8
n=15: 11.5, 13.0, 14.8
I understand why there are differences the way smoothness is computed
make estimate different (partial derivatives for each variable x,y,z
but on different sample size)
3) Search volume
n=42 S= 536352
n=15 866784 (subset of the n=42 same threshold etc)
It is more difficult to understand the large differences (is this a
problem with the thresholding of my data) or a different correction
(weighting) applied by SPM99 to the n=42 versus n=15. Basically is
it a physical reality or a construct and how is it done.
4) Resels
n=42 R=205
n=15 R=334
Should we report all these results when we compare groups (42 vs 15) ?
(since results between different research labs can be exchanged
(reproduced) if all variables are known)
How does the search volume and the computation of the smoothness(
FWHM in xyz) differ between SPM96 and SPM99 since I found differences
between both although statistical results are almost identical.
Thank you for your help
Didine
--
Badreddine Bencherif, MD
Department of Radiology
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: (410) 283-2050
Fax: (410) 614-1977
email: [log in to unmask]
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