Dear Steve:
I believe spm99 has now fixed this issue so that the user does not
have to worry about whether they are entering a conjunction or not.
SO, if you are looking at a conjunction and want an uncorrected p
value simply enter .001 (if you want that as your threshold), and
spm99 figures out how many comparisons are in the conjunction and
adjusts it accordingly.
In spm99b the threshold was computed as
u = spm_u(u,edf,STAT); for u<1, where u is your p value.
in spm99 the threshold is now computed as
u = spm_u(u^(1/n),edf,STAT); where u is again p value and n is the
number of contrasts in your conjunction. (line 630).
So if you're using .001^(1/12) you really are thresholding it at .562
uncorrected in spm99.
regards,
Darren
>SPMers,
>
>I'm having some trouble figuring out a proper threshold for a
>conjunction analysis. I gather from previous postings that a
>conjunction threshold is usually the p value that one would use on a
>single contrast, raised to the power of 1/number of contrasts going
>into the conjunction. See
>(http://www.mailbase.ac.uk/lists/spm/1999-07/0033.html)
>
>So for my particular case I'd like to use an individual level p of
>0.001, in a conjunction of 12 contrasts which means my conjunction
>level p is 0.001^(1/12) = 0.562.
>
>The problem is that the map I get at this conjunction threshold
>looks as though it was made using a much less stringent threshold,
>and does not reflect the activation seen in individual contrasts at
>0.001. In fact, virtually the whole brain is shown as active. Also
>if I use 0.01 rather than 0.562, the map looks reasonable.
>
>Is the new version of SPM99 correcting the p value entered for
>conjunction analysis, or am I calculating the p value incorrectly?
>
>Thank you for your help,
>
>Steve
Darren R. Gitelman, M.D.
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