Dear Didine,
> In a large group of n=42 I have
> search volume = S= 536352
> fwhm 11.5, 13.2, 13.8,
> 205.5 resels= R
>
> a subset of 15 subjects (of the 42)
> S= 651480
> fwhm 11.2, 12.3 , 13.2,
> 294.6 resels
>
> 1) any explanation for the large difference in S. Data are identical
> and I use the same threshold for both.
Joe already answered this. There are several masks SPM applies. Take
also a look at
http://www.jiscmail.ac.uk/cgi-bin/wa.exe?A2=ind0006&L=spm&P=R13693
> 2) Is there a way to constrain the search volume (since FWHM are
> almost similar) to make comparison more adequate (since p value is
> proportional to the number of resels (V/fwhm x y z ) or is it
> unreasonable.
>
> In other words how much a change from 200 to 300 resels will
> influence p values.
The estimated p-value is also a function of the t-value and the resel
count, therefore it is difficult to answer your question. However, have
a look at the function spm_P.m. This is the function that implements the
SPM99 p-value estimation (Worsley KJ et al 1996, Hum Brain Mapp.
4:58-73). By experimenting with the inputs you can answer your question
quite easily.
Stefan
--
Stefan Kiebel
Functional Imaging Laboratory
Wellcome Dept. of Cognitive Neurology
12 Queen Square
WC1N 3BG London, UK
Tel.: +44-(0)20-7833-7478
FAX : -7813-1420
email: [log in to unmask]
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