CCV or Additive error models?

Dear nmusers, In one of my PK analysis, if I chose Y=F+F*ERR(1)+ERR(2), I got lower objective value=1130.907, but $COV step failed (S matrix is singular); when I chose Y=F+ERR(1), I got $COV successful but higher objective value=1351.735. I prefer to choose the addivtive error model even if with higher objective value because it seems comfortable to see $COV successful. Can anyone give me other suggestions? Thanks Yuanyue (Paul) Gao School of Pharmacy University of Pittsburgh 716 Salk Hall 3501 Terrace Street Pittsburgh, PA 15261 Phone: 412-648-8546 E-mail: [email protected]