Dear Chung-Haow Tu,
At 19:50 27/05/99 +0800, Chung-Haow Tu wrote:
| Other, I noticed that there is a new model "Multi-subject:
| cond. x subj. interact & covaries" in SPM99. What's the model
| estimate for? I'm confused by the term "subj. interact".
This model is a multi-subject model where conditions are modelled in a
subject specific manner, i.e. via condition by subject interaction terms.
That is, each subject has their own condition effects modelled.
(This model is the same as would hav been acheived in SPM96 by choosing
"Multi-study: conditions ..." and entering each subject as their own
study.)
Note that the subject by condition interaction is still treated as a fixed
effect. To treat it as a random effect you would need to write out the
contrast images for each subject and use a second level analysis. This
model is ideal for that (and is suitable for looking at sample average
effects and conjunctions across subjects), because the design is
subject-separable: I.e. the parameter estimates for each individual subject
are independent of those for other subjects.
| Is that mean this model can deal with some subject specificity
| covaries like gender?
Subject specific covariate fits, in SPM terminology, refers to fitting
covariates via their interaction with subject. Subject specific global
AnCova is an example, where each subject gets their own slope for the
regression on the global mean.
Gender is a factor. You cannot examine main effects of subject specific
factors in a repeated measures design, because such effects are confounded
with subject block effects. For things such as gender which stratify the
subjects into groups, you can only look at group by condition interactions.
Even then, the appropriate analysis for the desired generalisation (to the
sample or population) must be used.
Hope this helps,
-andrew
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