help > NBS ANCOVA 3 Group Contrasts
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Jan 5, 2021  10:01 PM | Jessica Hua
NBS ANCOVA 3 Group Contrasts
Hi NBS Experts,

I had a question regarding contrasts in my study design. My study includes three groups: HC, PR, SCZ and the covariate is age. My abbreviated design matrix is:

1  1  0  21
1  1  0  22
1  1  0  24
1  1  0  20
...
1  0  1  17
1  0  1  16
1  0  1  18
1  0  1  21
...
1  0  0  22
1  0  0  22
1  0  0  18
1  0  0  24
...

To examine an overall difference among groups would my contrast be [0 1 1 0]? And how would I write the contrasts to compare HC > SCZ, HC > PR, PR > SCZ?

Thank you,

Jessica
Jan 6, 2021  11:01 AM | Andrew Zalesky
RE: NBS ANCOVA 3 Group Contrasts
Hi Jessica,

To test for a difference between any of the three groups (i.e. three-way ANOVA), the first column of the design matrix should only include 1's for the first group (rather than all groups). The contrast would then be [1 1 1 0]. Select F-test.

That said, I think that your current design and the contrast of [0 1 1 0] would give the same result. You can check this for yourself.

Three independent t-tests can be used to asses specific pairs of groups. The NBS Manual and Help section provides examples of a t-test.

Another option is to perform post-hoc t-test just on the connections that were identified with the initial F-test. This is a bit more complicated and would require you to extract the connectivity values from each edge comprising the significant network.

I hope that helps and I would be glad to answer any further questions!

Best wishes,
Andrew
Originally posted by Jessica Hua:
Hi NBS Experts,

I had a question regarding contrasts in my study design. My study includes three groups: HC, PR, SCZ and the covariate is age. My abbreviated design matrix is:

1  1  0  21
1  1  0  22
1  1  0  24
1  1  0  20
...
1  0  1  17
1  0  1  16
1  0  1  18
1  0  1  21
...
1  0  0  22
1  0  0  22
1  0  0  18
1  0  0  24
...

To examine an overall difference among groups would my contrast be [0 1 1 0]? And how would I write the contrasts to compare HC > SCZ, HC > PR, PR > SCZ?

Thank you,

Jessica