Dear Grant,
>I need help in interpreting the output of an analysis with a co-variate of
>interest. I want to determine the correlation between the difference in PET
>images between two conditions to the difference in a self-report covariate
>collected during each condition. My question is how can the Z scores given in
>the Output table be converted into correlation coefficients.
>A standard Fisher Z score to R table yields correlation coefficients that seem
>overly high. For example, a Z score of 2.0 yields a correlation
coefficient of
>0.965. That is pretty high for a behavioral co-variate, and some of the
>Z-scores are > 4.0, which would be r > 0.999 !
>Would it be more appropriate to take either the corrected or uncorrected (for a
>priori regions) p-value and then use that to find the correlation coefficient,
>using the number of subjects - 2 for the degrees of freedom. Under these
>conditions, a p-value of 0.01 (corrected) and 7 subjects would equal to r =
>0.8745, which seems more reasonable.
The Z-scores SPM reports cannot easily be transformed into correlation
coefficients. It is easier to get the adjusted data and do it post-hoc: Go
to results and plot the adjusted data. You will then find a variable y in
Matlab workspace. This matrix contains the adjusted (for globals, subject
effects) data. You can now look for the correlation between y and your
behavioural data.
-Christian
Dr Christian Büchel
Functional Imaging Laboratory
Wellcome Department of Cognitive Neurology
12 Queen Square
London WC1N 3BG, UK
Tel.:+44-171-833-7483
Fax.: +44-171-813-1420
email: [log in to unmask]
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