adding error term to covariate

Dear group, I have a baseline as covariate in the model, and it is the same measurement as the modeled variable. To me, it is reasonable to believe that there is measurement error for this covariate. Is there a way to incorporate some kind of error term into this baseline covariate? Thanks a lot. LI,HONG Graduate student University of Florida School of Pharmacy Department of Pharmaceutics Office: P4-10 Phone: (352)273-7865