> My question is how could I estimate those initial values so that the nls > fitting works. > You can't. Your parameters are almost certainly nonidentifiable (which is what Gabor told you more gracefully).
Just because you believe in a complex (often mechanistic) nonlinear model and have some data does not assure that the model parameters can be estimated. If you do not understand why this is so, consider fitting even a simple 4 parameter logistic when the data do not level off at the top and/or bottom end. There are then infinitely many solutions in which the parameters "trade off" with one another to give essentially identical fits. That is what the "singular gradient" message is trying to tell you. Bert Gunter Genentech Nonclinical Statistics ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
