Hi Anna:
Proving a negative is never easy but in your case it is quite clear
you have failed to succeed!
The question is not "Are these activations there by chance?" but
rather "Do I have sufficient evidence to reject the hypothesis of
activation?".
Normally the intrepid researcher must protect against false positive,
hence the statement that result X has less than a 1-in-20 (p<0.05)
chance of being a false poitive.
Your aim, however, is the reverse and you must protect against false
negative and be able to state that the absence of result X has less
than a 1-in-20 chance of being a false negative.
Exactly how this might be achieved I am not sure (others may want to
pitch in here) but with activation at a Pun<0.001 I do not see how
you can possibly fulfill the criteria above.
Hope that helps...
Stuart.
---- Begin Original Message ----
From: Anna Barnes <[log in to unmask]>
Sent: Thu, 6 Dec 2001 16:10:28 -0500
To: [log in to unmask]
Subject: thresholds and a priori hypotheses
If you have a hypothesis that there are no differences between one
brain
scan and another in a group and you set your thresold at the a priori
hypothesis level of Pu<0.001 expecting there to be a blank SPM(t) map
and
this map shows blobs at this significance level what does that mean
as far
as your a priori hyopthesis of no difference goes. Are the blobs
real or
are they there just by chance ?
Best regards
Anna
Anna Barnes (PhD)
North Shore- LIJ Research Institute
Functional Imaging Laboratory - Centre for Neuroscience
New York University School of Medicine
350 Community Drive
Manhasset, NY 11030
Phone: (516) 562-2498
Fax: (516) 562-1008
---- End Original Message ----
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