Hi, NM users: I have a typical 2-compartment model that can describe my data quite well, except the final gradient for the additive residual error is '0'. I therefore fix the additive residual error to '0', but then NM would not run. I tried different initial estimations for other parameters but the additive residual error seems to be the one that decide whether NM will run. Can anyone tell me why this would happen and how to solve it?
Thank you very much in advance for your help. Siwei
