3dMVM omnibus F-test with 3 groups

AFNI Message Board
1

Hi,

Im trying to compare FC of 3 groups using 3dVMV. I tried using this glfLabel below for the F-test, but get the following error: Error in glfRes[[ii]]$Df : $ operator is invalid for atomic vectors.

Heres a portion of my script. Could you let me know where the error is?

Thank you.

-bsVars ‘group+age+sex+FD’
-qVars ‘age,FD’
-SS_type 2
-num_glf 1
-glfLabel 1 ‘omnibusF’ -glfCode 1 ‘group : 1short -1intermediate & 1short -1long & 1intermediate -1long’
-resid /resid_3MVM_DMN.nii.gz
-dataTable
Subj group age sex FD InputFile
Subject124 short 30 2 0.195394 /BETA_Subject124_Condition001_Source004.nii
…
Subject127 long 22 2 0.113489 /BETA_Subject127_Condition001_Source004.nii
…
Subject130 intermediate 19 1 0.1608 /BETA_Subject130_Condition001_Source004.nii \

2

3dMVM automatically provides an omnibus F-statistic for the comparisons among the three groups. In addition, your -glfCode specification is rank deficient - only two of the three are needed.

3

I see. So, If I remove one of the three contrasts it would show me a similar omnibus test? I could use gltCode to compare two groups as well like a post-hoc test, right?

Thank you!

Gang Wrote:

3dMVM automatically provides an omnibus
F-statistic for the comparisons among the three
groups. In addition, your -glfCode specification
is rank deficient - only two of the three are
needed.

4

If I remove one of the three contrasts it would show me a similar omnibus test?

Yes, it should.

I could use gltCode to compare two groups as well like a post-hoc test, right?

That’s right. You can also specify the omnibus test for the three groups using gltCode since it’s a test with one degree of freedom:

-gltLabel 1 ‘omnibusF’ -gltCode 1 ‘group : 1short -1intermediate & 1short -1long’ \