[ferret_users] statistical significance, file reading

[nuncio@xxxxxxxxx]

Hi all,
          I am trying to do a statistical significance test using ferret.  I made 3 files for this purpose, one the degrees of freedom (dof) for a region, second one is the t-values(tval) and the third one is the t-table with dimensions of significance levels(x) and degrees of freedom (y axis).  Now I want to find the significance levels from the third file based on the values in the first two files. I tried it this way, 
use dof file
use t value file
use t table file
let signif=pp[d=3,y=`dof[d=1]`,x=@loc:`tvalue[d=2]`]
ferret gives an error 'grave evaluates to scalar'
Is there anyother way to do it
Thanks 
nuncio