On Tue, 31 Aug 2004 21:17:33 +0100, X Liu <[log in to unmask]> wrote:
>Concatenate run/session or not?
>
>We have a design with 2 independent variables (IV1 and IV2), each with 3
>levels. IV1 is manipulated across blocks within a run and IV2 is
>manipulated across runs. There are two approaches I can think of to anaylze
>the data for the main effects and interaction for this design. What is the
>advantage and disadvantage of each approach, from the conceptual and
>practical perspectives? Is one more valid than the other from the
>statistics point of view? Thanks very much.
>
>
>Approach 2:
>Analyze each run separately and model just the 3 conditions/levels of IV1
>(ev1, ev2, ev3). Then the main effects of IV1 can be set up as below.
>
>Contrast ev1 ev2 ev3
>mean (1) 1 1 1
>IV1 (2) 1 -1 0
> (3) 1 0 -1
> (4) 0 1 -1
>
>And then proceed to the group analysis with the EVs/contrasts setup as
>below for the three levels of IV2 (say for 5 subjects)
>
>Group ev1 ev2 ev3 ev4 ev5 ev6 ev7 ev8
>1 1 1 1 1 0 0 0 0
>1 1 1 1 0 1 0 0 0
>1 1 1 1 0 0 1 0 0
>1 1 1 1 0 0 0 1 0
>1 1 1 1 0 0 0 0 1
>1 1 -1 0 1 0 0 0 0
>1 1 -1 0 0 1 0 0 0
>1 1 -1 0 0 0 1 0 0
>1 1 -1 0 0 0 0 1 0
>1 1 -1 0 0 0 0 0 1
>1 1 0 -1 1 0 0 0 0
>1 1 0 -1 0 1 0 0 0
>1 1 0 -1 0 0 1 0 0
>1 1 0 -1 0 0 0 1 0
>1 1 0 -1 0 0 0 0 1
>
>Contrast ev1 ev2 ev3 ev4 ev5 ev6 ev7 ev8
>c1 1 0 0 0 0 0 0 0
>c2 0 1 0 0 0 0 0 0
>c3 0 0 1 0 0 0 0 0
>c4 0 1 -1 0 0 0 0 0
>
>Then the .gfeat folder should include 4 cope#.feat subfolders, one for each
>of the contrasts from the first level. zstat2 to zstat4 of cope1.feat will
>assess the main effect of IV2 (zstat1 is the overall grand mean of both IVs
>again baseline). zstat1 of cope2.feat to cope4.feat will assess the main
>effect of IV1. zstat2 to zstat4 of cope2.feat to cope4.feat will assess the
>interactions.
Just found out an error. To correct myself, ev1 is a linear combination of
ev4 to ev8. So take out ev1 and contrast 1 is just the combination of the
new ev3 to ev7 (weighted or not WRT to the other contrasts?). The
interpretation of the main effects and interaction above stands.
Group ev1 ev2 ev3 ev4 ev5 ev6 ev7
1 1 1 1 0 0 0 0
1 1 1 0 1 0 0 0
1 1 1 0 0 1 0 0
1 1 1 0 0 0 1 0
1 1 1 0 0 0 0 1
1 -1 0 1 0 0 0 0
1 -1 0 0 1 0 0 0
1 -1 0 0 0 1 0 0
1 -1 0 0 0 0 1 0
1 -1 0 0 0 0 0 1
1 0 -1 1 0 0 0 0
1 0 -1 0 1 0 0 0
1 0 -1 0 0 1 0 0
1 0 -1 0 0 0 1 0
1 0 -1 0 0 0 0 1
Contrast ev1 ev2 ev3 ev4 ev5 ev6 ev7
c1 0 0 0.2 0.2 0.2 0.2 0.2
c2 1 0 0 0 0 0 0
c3 0 1 0 0 0 0 0
c4 1 -1 0 0 0 0 0
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