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.
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