Dear SPM Listmembers:
I have a few questions of clarification.
1) Regarding SPM contrasts.... Since contrasts need to be orthogonal, I
would assume this means each contrast uses unique sum of squares
(what some statistical programs like SAS refer to as Sum of Squares III)
and, therefore, the F-statistics for a given contrast take into account
(partition out) the variance associated with the other contrasts. Is this the
case???
2) Regarding construction of covariate vectors...
I have a covariate of interest related to accuracy of performance. The
accuracy score was only available and collected during one condition. I
plan to use the PET multi-subject, different conditions, covariate design for
analysis. Can the covariate vector be constructed with zeros when the
accuracy score is not available as in Condition 2 below???
Subject # Experimental Condition/Scan Covariate Vector
(Accuracy Score)
01 Condition 1, Mean Brain Volume 11.3
01 Condition 2, Mean Brain Volume 0
02 Condition 1, Mean Brain Volume 18.6
02 Condition 2, Mean Brain Volume 0
03 Condition 1, Mean Brain Volume 15.4
03 Condition 2, Mean Brain Volume 0
etc. for 6 subjects
I am concerned that the covariate vector is closely related to the
expeimental condition column. Should I transform the covariate vector so
that it sums to zero by subtracting the mean score from each value (this
would still preserve the relative rank of the subject's accuracy scores)????
I would then test two contrasts reflecting the condition 1 and the
covariate effects.
Condition1 Condition2 Covariate
Contrast 1 1 -1 0
Contrast 2 0 0 1
Will contrast 2 then reflect the activation associated with the accuracy
of performance during condition 1 ???
Thanks for your comments.
Joe Tracy
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