Hello SPMers,
How do I compute the contrasts to implement a 1-sample and a 2-sample test in SPM?
I have set up a 3-by-2 Flexible Factorial ANOVA in SPM8. For each subject, I have 2 contrast images, one for each condition, for the ANOVA. I used the paper by Glascher and Gitelman (http://www.sbirc.ed.ac.uk/cyril/downloads/Contrast_Weighting_Glascher_Gitelman_2008.pdf) as a guide to set up the ANOVA.
Factors were set up as follows:
Factor 1: subject independence – yes, variance – equal
Factor 2: group independence – yes, variance – unequal
Factor 3: condition independence – no, variance – equal
F-Contrasts were set up as follows:
n1 = 12; (number of subjects in group 1)
n2 = 10; (number of subjects in group 2)
n3 = 7; (number of subjects in group 3)
nc = 2; (number of levels in condition factor)
ng = 3; (number of groups)
MEg = [1:nc]-mean(1:nc); (main effect of group, here: [-1 0 1])
MEc = [1 -1]; (main effect of group: Condition 1 > Condition 2)
Main effect of Group:
MEg zeros(1,nc) -ones(1,nc)/nc zeros(1,nc) ones(1,nc)/nc -ones(1,n1)/n1 zeros(1,n2) -ones(1,n3)/n3
Main effect of Condition:
zeros(1,ng) MEc MEc*[n1/(n1+n2+n3)] MEc*[n2/(n1+n2+n3)] MEc*[n3/(n1+n2+n3)] zeros(1,n1+n2+n3)
Group-condition interaction;
zeros(1,ng) zeros(1,nc) -MEc zeros(1,nc) MEc zeros(1,n1+n2+n3)
Note that unlike in SPM5, the design matrix SPM8 positions the subject factor after the group factor, condition factor, and interactions.
Using the above setup, I have results for the main effects and interactions of my study, and am satisfied with the results.
But I would like to use 1- and 2-sample t-tests post hoc to further examine the relationships between groups and conditions. I have used the built-in options in SPM8 for these t-tests, but I have been informed that t-test within the Flexible Factorial framework lead to better results because variances are pooled from all groups in the study. I tried a 1-sample t-test the following way- to test for Condition 1 > Condition 2 in Group 1 - and got an error:
1 0 0 MEc*[1/nc] MEc*[1/nc] zeros(1,nc) zeros(1,nc) ones(1,n1) zeros (1,n1) zeros(1,n1). What would be the correct contrast for such a 1-sample t-test?
Any help is greatly appreciated. Thank you very much in advance!
Kwaku Akrofi, PhD
Postdoctoral Fellow
Department of Speech and Hearing Science
University of Illinois at Urbana-Champaign
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