These appear to be primarily statistics/nonlinear optimization issues that are off topic here, which is about R programming. Post on a statistics list like stats.stackexchange.com instead.
Cheers, Bert Bert Gunter "Data is not information. Information is not knowledge. And knowledge is certainly not wisdom." -- Clifford Stoll On Tue, Aug 18, 2015 at 9:08 AM, Xochitl CORMON <xochitl.cor...@ifremer.fr> wrote: > Dear all, > > I am trying to estimate VBGF parameters K and Linf using non linear > regression and nls(). First I used a classic approach where I estimate both > parameters together as below with "alkdyr" being a subset per year of my > age-length-key database and running in a loop. > > vbgf.par <- nls(Lgtcm ~ Linf *(1 - exp(-K * (Age - tzero))), start = c(K= > 0.07, Linf = 177.1), data=alkdyr) > > I obtain an estimation of both parameters that are strongly correlated. > Indeed after plotting Linf ~ K and fitting a linear regression I obtain a > function (Linf = a + b*K) with R2= 0.8 and a = 215, b = -763. > > In this context, to take into account explicitly correlation between > parameters, I decided to fit a new non linear regression derivate from VBGF > but where Linf is expressed depending on K (I am most interested in K). To > do so, I tried this model: > vbgf.par <- nls(Lgtcm ~ (a + (b*k)) *(1 - exp(-k * (Age - tzero))), start = > c(k= 0.07, a= 215, b=-763), data=alkdyr) > > Unfortunately at this point I cannot go further as I get the error message > "singular gradient matrix at initial parameter estimates". > > I tried to use alg= plinear (which I am not sure I understand properly yet). > If I give a starting value for a and b only, I have an error message stating > "step factor below minFactor" (even when minFactor is set to 100000000000). > > Any help will be more than welcome as this is quite urgent.... > > Best, > > Xochitl C. > > > > > -- > > <>< <>< <>< <>< > > Xochitl CORMON > +33 (0)3 21 99 56 84 > > Doctorante en écologie marine et science halieutique > PhD student in marine ecology and fishery science > > <>< <>< <>< <>< > > IFREMER > Centre Manche Mer du Nord > 150 quai Gambetta > 62200 Boulogne-sur-Mer > > <>< <>< <>< <>< > > ______________________________________________ > R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see > 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. ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see 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.
