We are preprocessed our data to perform a longitudinal study using longitudinal pipeline of Freesurfer 4.5. However, when we try to do the group-analysis using qdec (Freesurfer 5.1), we obtain the following error:

ERROR: QdecGlmDesign::GenerateContrasts: factor 1 must have 2 levels
ninputs = 64
Checking inputs
nframestot = 64
Allocing output
Done allocing
nframes = 64
Writing to /home/barbara/subjects/qdec/Untitled/y.mgh
gdfReadHeader: reading /home/barbara/subjects/qdec/Untitled/qdec.fsgd
INFO: gd2mtx_method is dods
Reading source surface /home/barbara/subjects/fsaverage/surf/lh.white
ERROR: no contrasts specified.
Error in Analyze: command failed: mri_glmfit --y /home/barbara/subjects/qdec/Untitled/y.mgh --fsgd /home/barbara/subjects/qdec/Untitled/qdec.fsgd dods --glmdir /home/barbara/subjects/qdec/Untitled --surf fsaverage lh

Has anyone experienced a similar problem and is there any way to correct it?

Best,