t-test degrees of freedom

Hi you all,

I am using Matlab Version 2019a, and GraphVar 2.02. I am calculating group comparisons with 4 subgroups that arise when combining 2 factors (factor A and B) so that I am calculating e.g., A0 + B0 vs. A1 + B0. Hence, I have 6 comparisons in total. My total n is 211, but my subgroups have an n of 60, 47, 24, and 80, respectively.

Upon analyzing the output, I encountered two difficulties:

 

(1) When I it understand correctly, the t value should indicate whether one group or the other has e.g., nodes with greater strength in a specific brain region. If we have positive values, the first group (e.g., A0 +B0) has greater values, if the t-value is negative ,the second group (e.g., A1 +B0) does. Why does the sign of the t-value differ for one brain region depending on the threshold?

 

(2) Why are the df 207 and not, e.g., (60+47) -2 = 105 for the t-test? As we are comparing separate groups and not the whole sample, I do not understand why the df is caculated 211 (total n) - 4 (number of groups)? If I am not mistaken, this is not the way df for t-tests are calculated?

 

Thank you so much for making this toolbox available! I am looking forward to your answers! :)

 

Best,

Marieke

 

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