[R-sig-eco] reprametrization of Lloyd-Taylor function

[Zoltan Botta-Dukat]

Dear All,

I tried to fit Lloyd-Taylor function for soil respiration data, but 
convergence is often failed and even if the model converged there is 
extremely high uncertainty in parameter estimates.

The fitted model is: y=a-b/(x-c), where a, b, and c are the parameters. 
(Note that it is not the original Lloyd-Taylor function, in my version 
the dependent variable is the logarithm of respiration).

I have already recognized the origin of the problem: since c<min(x), 
partial derivatives of y with respect to b and c are highly correlated. 
I suppose that it can be solved by an appropriate re-parametrization, 
but I was not able to find it. Could you suggest an appropriate 
re-parametrization?

Thanks

Zoltan


-- 
Botta-Dukát Zoltán
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Ökológiai és Botanikai Intézet
Magyar Tudományos Akadémia
Ökológiai Kutatóközpont
--------------------------------
2163. Vácrátót, Alkotmány u. 2-4.
tel: +36 28 360122/157
fax: +36 28 360110
botta-dukat.zoltan at okologia.mta.hu
www.okologia.mta.hu


Zoltán BOTTA-Dukát
--------------------------------
Institute of Ecology and Botany
Hungarian Academy of Sciences
Centre for Ecological Research
--------------------------------
H-2163 Vácrátót, Alkomány u. 2-4.
HUNGARY
Phone: +36 28 360122/157
Fax..: +36 28 360110
botta-dukat.zoltan at okologia.mta.hu
www.okologia.mta.hu