Hi Andrew,

I have a question regarding my design matrix set up and have mostly zero values when extracting connectivity strengths in the significant sub-network.

In a between-subjects design, I want to use an ANCOVA design to test the interaction between 2 bivariate variables; diagnosis (2 groups) and trauma (2 groups) co-varying for the effect of sex and age. An example of the design matrix I have used to test for this interaction between diagnosis and trauma is illustrated below. The contrast I used in NBS to test this interaction was set up as: structural connectivity matrices -weighted-by FA (.txt) "000100" F-test, 5000 permutations using extent. Do you think that this design matrix set up is correctly to test this interaction between diagnosis and trauma?

Columns: Intercept (1), Diagnosis (HC=1, SZ=-1), No Trauma (1)/Trauma (2), Interaction (1, -1, 2, -2), Sex (1, -1), Age

1 1 1 1 -1 45
1 1 2 2 -1 47
1 -1 1 -1 -1 44
1 -1 2 -2 -1 21
1 -1 1 -1 -1 34
1 -1 1 -1 -1 43

When I ran this F-test to test for an interaction effect, I obtained a sub-network at a threshold of 8.5. I then tried to extract the connectivity strengths between each connection in the sub-network using the matlab script previously recommended in the NBS forum and manual. However, the connectivity strength values that I obtain come out as '0' values in most cases. For reduced design using this same data set, I am able to extract connection strengths that appear reasonable for a F-test contrast investigating the main effect of diagnosis that is modelled as '0100'.

Below is an example of the output I receive in the matlab window after running the script to extract connection strengths for a sub-network relating to the interaction between diagnosis and trauma consisting of 26 edges, and 25 nodes in (T=8.5)
0
0.298
0
0
0
0
0
0
0
0.273
0
0
0
0
0
0
0
0
0
0.513
0
0

Do you know what might be causing these 0 values in the connectivity strengths to appear? Is it possible that there could be a limit to the extent of the statistical model/ design that is suitable for this approach? Would you have any suggestions how we might overcome the issue or indeed whether you think the sub-network is valid?

Thank you for your help in advance.
Kind Regards
Laura"