Hello, I am trying to model a type II functional response of number of prey eaten (Ne) against number supplied (No) with a non-linear least squares regression (nls). I am using a modification of Holling's (1959) disc equation to account for non-replacement of prey;
Ne=No{1-exp[a(bNe-T)]} where a is the attack rate, b is the handling time, and T is the experimental period. My script is as follows; model<-nls(Ne~No*(1-exp(a*(b*Ne-T))),start=list(a=1.2,b=0.015,T=24)) This returns the following error; "Error in nlsModel(formula, mf, start, wts) : singular gradient matrix at initial parameter estimates". Can anyone please advise as to what I may be doing wrong? I have tried modifying the parameter start points with no success. Many thanks, -- View this message in context: http://r.789695.n4.nabble.com/NLS-Singular-Gradient-Error-tp2069029p2069029.html Sent from the R help mailing list archive at Nabble.com. ______________________________________________ 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.