The pathstats.byvoxel.txt files will generally not contain the same number of positions (rows) for each subject because the tracts are reconstructed in each subject's native diffusion space and not in a template space. Thus they are not ready for performing group analyses yet. To combine these files from multiple subjects, interpolating the anisotropy and diffusivity values at corresponding positions along the tract for all subjects, run the following:

trac-all -stat -c $TUTORIAL_DATA/diffusion_tutorial/dmrirc.tutorial

However, when I try to use the -stat option, it says it is not recognized. 
There is no -stat option described in the help, either.