Dear Ged and other SPM-users
I have noticed the same phenomena that you point out at "*" below.
The negativeness of the fourth EC-density makes the FWE-corrected
p-value to become low in a short intervall of small t-values when it
should obviously be 1 (about from t=0.7 to t=1). Perhaps one should
set EM = max(0,EM)? It is problably not a big problem since voxel
thresholds shouldn't be that low, but it might affect other
applications of the spm_P_RF. I noticed it when making a function for
conjunction by maximizing p-values.
At 10:57 2007-04-16, Ged Ridgway wrote:
>Dear RFT experts,
>
>I've noticed that for a couple of VBM studies with normal-looking
>FWE-corrected results, when I came to check xSPM.R I saw that the
>first two resel counts are negative.
>
>While it seems reasonable (or at least possible) that the Euler
>Characteristic might be negative, I'm confused as to how the caliper
>diameter (xSPM.R(2)) could possibly be negative. Any comments?
>
>It seems possible from spm_P_RF that negative resel counts might
>produce a negative Em,* which produces a NaN from spm_Pcdf. This
>might possibly be the cause of a later error. E.g. maybe in this
>apparently unanswered post:
> http://www.jiscmail.ac.uk/cgi-bin/webadmin?A2=ind06&L=SPM&P=R775760
>
>Perhaps line 106 of spm_P_RF should be replaced with something like
> lam = max(eps, Em*p);
> P = 1 - spm_Pcdf(c - 1, lam);
>
>to return p-values of 1 instead?
>
>*(also, the fourth EC density at line 160 of spm_P_RF can be
>negative for t less than 1 -- or near 1, and small DF, e.g.
> v = 10; t = 1.05;
> (v-1)*(t.^2)/v - 1 < 0
>It's probably daft to ask for the corrected p-value of such low
>t-values, but I would guess that it would be better to return p=1 than p=NaN)
>
>All the best,
>Ged.
/Micael
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