[Freesurfer] two questions regarding qdec and conjunction analysis is FS

Dear FS experts,

I have two questions:

1. In qdec.table.dat file I am specifying several continuous factors, e.g factor1 and factor2. For some subjects factor2 is missing, therefore I leave empty space in those cells. When I load a qdec.table.dat into qdec, the whole line of the subject with missing factor2 is omitted. I was wondering if I can make some input into the empty cell, so that qdec recognizes that for this subject the value is missing only for factor2, and the value for factor1 is present.

2. Is it possible to do conjunction analysis in FreeSurfer, I mean looking at common differences in two groups as compared to the third group?

Thank you for your time and help, Tanja.