Dear SPM'ers,
In the second level of a random analysis with PET data, I have three conditions A, B, and C:
Conditions B and C are well matched except for cognitive components of interest (CCI). However, condition A differs unfortunately from conditions B and C for unspecifics aspects (stimulus presentation mode). I posit that the CCI involved in condition A is also involved in condition B, but not in condition C.
Hence, subtraction [ A - B ] will remove the CCI and highlight only brain structures related to unspecific effects, while subtraction [ A - C ] will highlight both CCI and unspecific effects.
Since I want to segregate the CCI from unspecifics effects in the context of condition A (otherwise B - C was sufficient), I perform the subtraction [ A - C ] masked exclusive by [A - B ] at p = 0.05.
I wonder about the validity and the interpretative limitations of this procedure, given that I cannot assume the independence between the two subtractions. In one sense, it looks like an interaction, but I strongly suspect that it do not have the real taste of an interaction ...
Any comment and eventual alternative proposals will be greatly appreciated.
All the best and many thanks
Philippe
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PEIGNEUX Philippe, Lic. Psych., Chercheur
E-mail: [log in to unmask]
Cyclotron Research Centre
Neurology Unit & Neuropsychology Department
Liege University BELGIUM
http://www.ulg.ac.be/neuropsy/
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