[R] Non linear regression - Von Bertalanffy Growth Function - "singular gradient matrix at initial parameter estimates"

Bert Gunter
2015-08-18T16:20:02Z

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.Cormon at 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 ?cologie marine et science halieutique
> PhD student in marine ecology and fishery science
>
> <>< <>< <>< <><
>
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>
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>
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