[Freesurfer] Qdec error and design problem

Dear all,

I am trying to run statistical analysis on cortical thickness data using Qdec. I have one group of control subjects that are split into three genotype groups. I want to investigate if cortical thickness significantly between the three genotype groups. Is it possible to look at differences between three groups using Qdec? I am getting the following error when I try to analyse - ERROR: QdecGlmDesign::Create: zero factors!

I would really appreciate if anyone could shed some light on this for me as this is the first time I have used Qdec and I don't know what is causing this error.

Thank you for your help.

Sinead